ACTA

UNIVERSITATIS

UPSALIENSIS

UPPSALA

2007

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 375

Photoelectron Spectroscopy onAtoms, Molecules and Clusters

The Geometric and Electronic Structure Studied bySynchrotron Radiation and Lasers

TORBJÖRN RANDER

ISSN 1651-6214ISBN 978-91-554-7047-0urn:nbn:se:uu:diva-8343

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“Och kallas därför höga bergen hos oss fjäll, av orsak att där oppå inga trän

eller örter växa, utan de är glatta och bara såsom fjäll uppå en fisk”

– Olof Rudbäck d. ä.

List of Papers

This thesis is based on the following papers, which are referred to in the text

by their Roman numerals.

I Experimental evidence for molecular ultrafast dissociation inO2 clustersT. Rander, M. Lundwall, A. Lindblad, G. Öhrwall, M.

Tchaplyguine, S. Svensson, O. Björneholm

Eur. Phys. J. D 42, 253-257 (2007)

II Suppression of the ultrafast dissociation of bromomethanemolecules in clustersT. Rander, A. Lindblad, A. Rosso, H. Bergersen, G. Öhrwall, S.

Svensson, O. Björneholm

In manuscript

III Core-level electron spectroscopy on the sodium dimer Na 2plevelT. Rander, J. Schulz, M. Huttula, A. Mäkinen, M. Tchaplyguine,

S. Svensson, G. Öhrwall, O. Björneholm, S. Aksela, H. Aksela

Phys. Rev. A 75, 032510 (2007)

IV The diffusion behavior of O2 doped on large Ar clustersT. Rander, A. Lindblad, M. Lundwall, M. Tchaplyguine, G.

Öhrwall, S. Svensson, O. Björneholm

Submitted to J. Chem. Phys.

V The far from equilibrium structure of argon clusters dopedwith krypton or xenonA. Lindblad, H. Bergersen, T. Rander, M. Lundwall, G. Öhrwall,

M. Tchaplyguine, S. Svensson, O. Björneholm

Phys. Chem. Chem. Phys. 8, 1899-1905 (2006)

5

VI The role of molecular polarity in cluster local structurestudied by photoelectron spectroscopyA. Rosso, T. Rander, H. Bergersen, A. Lindblad, M. Lundwall,

S. Svensson, M. Tchaplyguine, G. Öhrwall, L. J. Sæthre, O.

Björneholm

Chem. Phys. Lett 435, 79-83 (2007)

VII Shakedown in core photoelectron spectra from alignedlaser-excited Na atomsJ. Schulz, M. Tchaplyguine, T. Rander, O. Björneholm, S.

Svensson, R. Sankari, S. Heinäsmäki, H. Aksela, S. Aksela, E.

Kukk

Phys. Rev. A 72, 10702-1-4 (2005)

VIII Characterization of weakly excited final states by shakedownspectroscopy of laser-excited potassiumJ. Schulz, S. Heinäsmäki, R. Sankari, T. Rander, A. Lindblad,

H. Bergersen, G. Öhrwall, S. Svensson, E. Kukk, S. Aksela, H.

Aksela

Phys. Rev. A 74, 12705-1-6 (2006)

Reprints were made with permission from the publishers.

6

The following is a list of papers to which I have contributed, but which are not

included in this thesis.

Observation of elastic scattering effects on photoelectron angulardistributions in free Xe clustersG. Öhrwall, M. Tchaplyguine, M. Gisselbrecht, M. Lundwall, R. Feifel,

T. Rander, J. Schulz, R. R. T. Marinho, A. Lindgren, S. L. Sörensen, S.

Svensson, O. Björneholm

J. Phys. B 36, 3937-49 (2003)

Femtosecond Interatomic Coulombic Decay in Free Neon Clusters: LargeLifetime Differences between Surface and BulkG. Öhrwall, M. Tchaplyguine, M. Lundwall, R. Feifel, H. Bergersen, T.

Rander, A. Lindblad, J. Schulz, S. Peredkov, S. Barth, S. Marburger, U.

Hergenhahn, S. Svensson, and O. Björneholm

Phys. Rev. Lett. 93, 173401 (2004)

Final state selection in the 4p photoemission of Rb by combining laserspectroscopy with soft-x-ray photoionizationJ. Schulz, M. Tchaplyguine, T. Rander, H. Bergersen, A. Lindblad, G.

Öhrwall, S. Svensson. S. Heinäsmäki, R. Sankari, S. Osmekhin, S. Aksela,

H. Aksela

Phys. Rev. A 72, 32718-1-4 (2005)

The electronic structure of free water clusters probed by Auger electronspectroscopyG. Öhrwall, R. F. Fink, M. Tchaplyguine, L. Ojamae, M. Lundwall, R. R.

T. Marinho, A. N. de Brito, S. L. Sörensen, M. Gisselbrecht, R. Feifel, T.

Rander, A. Lindblad, J. Schulz, L. J. Sæthre, N. Mårtensson, S. Svensson, O.

Björneholm

J. Chem. Phys 123, 54310-1-10 (2005)

Ioniclike energy structure of neutral core-excited states in free KrclustersS. Peredkov, A. Kivimäki, S. L. Sörensen, J. Schulz, N. Mårtensson, G.

Öhrwall, M. Lundwall, T. Rander, A. Lindblad, H. Bergersen, S. Svensson,

O. Björneholm, M. Tchaplyguine

Phys. Rev. A 72, 21201-1-4 (2005)

7

Postcollision interaction in noble gas clusters: observation of differencesin surface and bulk line shapesA. Lindblad, R. F. Fink, H. Bergersen, M. Lundwall, T. Rander, R. Feifel, G.

Öhrwall, M. Tchaplyguine, U. Hergenhahn, S. Svensson, O. Björneholm

J. Chem. Phys. 123, 211101-1-4 (2005)

Enhanced surface sensitivity in AES relative to XPS observed in free ArclustersM. Lundwall, M. Tchaplyguine, G. Öhrwall, A. Lindblad, S. Peredkov, T.

Rander, S. Svensson, O. Björneholm

Surf. Sci. 594, 12-19 (2005)

Photon energy dependent intensity variations observed in Auger spectraof free argon clustersM. Lundwall, A. Lindblad, H. Bergersen, T. Rander, G. Öhrwall, M.

Tchaplyguine, S. Peredkov, S. Svensson, O. Björneholm

J. Phys. B 39, 3321-33 (2006)

Shell-dependent core-level chemical shifts observed in free xenon clustersM. Lundwall, R. F. Fink, M. Tchaplyguine, A. Lindblad, G. Öhrwall, H.

Bergersen, S. Peredkov, T. Rander, S. Svensson, O. Björneholm

J. Phys. B 39, 5225-35 (2006)

Lineshapes in carbon 1s photoelectron spectra of methanol clustersM. Abu-samha, K. K. Børve, L. J. Sæthre, G. Öhrwall, H. Bergersen, T.

Rander, O. Björneholm, M. Tchaplyguine

Phys. Chem. Chem. Phys 8, 2473-82 (2006)

Preferential site occupancy of krypton atoms on free argon-clustersurfacesM. Lundwall, A. Lindblad, H. Bergersen, T. Rander, G. Öhrwall, M.

Tchaplyguine, S. Svensson, O. Björneholm

J. Chem. Phys 125, 14305-1-7 (2006)

Laser excitation combined with 2p photoionization and Auger decay ofpotassiumK. Jänkälä, R. Sankari, J. Schulz, M. Huttula, A. Calo, S. Heinäsmäki, S.

Fritsche, T. Rander, S. Svensson, S. Aksela, H. Aksela

Phys. Rev. A 73, 22720-1-8 (2006)

8

Magnetron-based source of neutral metal vapors for photoelectronspectroscopyM. Tchaplyguine, S. Peredkov, H. Svensson, J. Schulz, G. Öhrwall,

M. Lundwall, T. Rander, A. Lindblad, H. Bergersen, S. Svensson, M.

Gisselbrecht, S. L. Sorensen, L. Gridneva, N. Mårtensson, O. Björneholm

Rev. Sci. Instrum. 77, 033106 (2006)

5p photoemission from laser-excited cesium atomsJ. Schulz, M. Määtä, S. Heinäsmäki, M. Huttula, R. Sankari, E. Kukk, T.

Rander, S. Svensson, S. Aksela, H. Aksela

Phys. Rev. A 73, 062721 (2006)

Self-assembled heterogeneous argon/neon core-shell clusters studied byphotoelectron spectroscopyM. Lundwall, W. Pokapanich, H. Bergersen, A. Lindblad, T. Rander, G.

Öhrwall, M. Tchaplyguine, S. Barth, U. Hergenhahn, S. Svensson, O.

Björneholm

J. Chem. Phys. 126, 214706 (2007)

Synchrotron radiation study of chloromethane clusters: Effects ofpolarizability and dipole moment on core level chemical shiftsA. Rosso, A. Lindblad, M. Lundwall, T. Rander, S. Svensson, M.

Tchaplyguine, G. Öhrwall, O. Björneholm

J. Chem. Phys. 127, 024302 (2007)

Free nanoscale sodium clusters studied by core-level photoelectronspectroscopyS. Peredkov, G. Öhrwall, J. Schulz, M. Lundwall, T. Rander, A. Lindblad,

H. Bergersen, A. Rosso, W. Pokapanich, N. Mårtensson, S. Svensson, S. L.

Sorensen, O. Björneholm

Phys. Rev. B 75, 235407 (2007)

Direct observation of the non-supported metal nanoparticle electrondensity of states by X-ray photoelectron spectroscopy M. Tchaplyguine,S. Peredkov, A. Rosso, J. Schulz, G. Öhrwall, M. Lundwall, T. Rander, A.

Lindblad, H. Bergersen, W. Pokapanich, S. Svensson, S. L. Sorensen, N.

Mårtensson, O. Björneholm

Eur. Phys. J. D 45, 295 (2007)

9

Localized versus delocalized excitations just above the 3d threshold inkrypton clusters studied by Auger electron spectroscopyM. Tchaplyguine,A. Kivimäki, S. Peredkov, S. L. Sorensen, G. Öhrwall, J. Schulz, M.

Lundwall, T. Rander, A. Lindblad, A. Rosso, S. Svensson, N. Mårtensson, O.

Björneholm

J. Chem. Phys. 127, 124314 (2007)

10

Contents

List of Papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Comments on my own participation . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1 Populärvetenskaplig sammanfattning . . . . . . . . . . . . . . . . . . . . . . . 15

1.1 De olika experimenten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.2 Experimentella metoder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.2.1 Elektromagnetisk strålning . . . . . . . . . . . . . . . . . . . . . . . 17

1.2.2 Elektronspektroskopi . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.3 Beräkningar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.4 Resultat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3 Fundamental concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1 The electronic structure of matter . . . . . . . . . . . . . . . . . . . . . . 23

3.1.1 Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1.2 Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.1.3 Clusters and solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.2 The geometric structure of matter . . . . . . . . . . . . . . . . . . . . . . 28

3.3 The photoelectric effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4 Experimental equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.1 Vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.2 Synchrotron radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.3 Laser Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.4 The MAX laboratory and beam-line I411 . . . . . . . . . . . . . . . . 37

4.5 Sample production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.5.1 Cluster production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.5.2 Doped cluster production . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.5.3 Metal vapor production . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5 Probing techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.1 Ultra-violet Photoelectron Spectroscopy (UPS) . . . . . . . . . . . . 43

5.2 X-ray Photoelectron Spectroscopy (XPS) . . . . . . . . . . . . . . . . 44

5.3 Auger Electron Spectroscopy (AES) . . . . . . . . . . . . . . . . . . . . 45

5.4 Near-edge Absorption Fine Structure (NEXAFS) . . . . . . . . . . . 46

5.5 Resonant Auger Spectroscopy (RAS) . . . . . . . . . . . . . . . . . . . 47

5.6 Interpretation of X-ray spectra . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.6.1 Koopmans theorem and the chemical shift . . . . . . . . . . . . 48

5.6.2 Atomic photoelectron spectra . . . . . . . . . . . . . . . . . . . . . 49

5.6.3 Molecular photoelectron spectra . . . . . . . . . . . . . . . . . . . 49

5.6.4 Cluster photoelectron spectra . . . . . . . . . . . . . . . . . . . . . . 50

6 Summary of papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.1 Dissociation of molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.1.1 Oxygen clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.1.2 Bromomethane clusters . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6.1.3 Sodium dimers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.1.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6.2 The structure of doped and molecular clusters . . . . . . . . . . . . . 61

6.2.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

6.3 Laser excited metal vapors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

6.3.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

7 Future outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

8 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

12

Comments on my own participation

Working in the field of experimental physics takes a lot of team-work, and

all of the papers in this thesis reflect this fact. Without the fruitful exchange

of ideas and without a helping hand running the experiments, not much of

the work presented here could have been performed. Common to all of the

included articles is that I have been actively involved in the experiments, and

in discussions regarding them. For the papers where I am the first author, I

was the main responsible for data analysis and manuscript preparation.

13

1. Populärvetenskaplig sammanfattning

Världen vi lever i, och allt som finns däri, består av atomer. En atom består av

en atomkärna, som är positivt laddad, och negativt laddade elektroner. Atom-

kärnan består i sin tur av ännu mindre enheter, så kallade neutroner och pro-

toner. De består i sin tur av ännu mindre byggstenar, men detta behöver vi

inte tänka så mycket på i vårt fall eftersom atomkärnan är väldigt liten (c:a

10000 gånger mindre än hela atoms storlek). I våra experiment kan vi där-

för behandla atomkärnan som en laddad punkt. Atomer hittar man ibland

tillsammans med andra atomer, i molekyler, kluster eller fasta material. Ett

kluster är en “liten” klump av atomer eller molekyler (3-50000 st), som sitter

ihop med varandra. Man brukar säga att ett kluster är ett system som länkar

samman förståelsen för egenskaperna hos en enskild atom eller molekyl, och

förståelsen för egenskaperna hos ett fast material. Denna avhandling handlar

om atomer, molekyler och kluster, och vad som kan hända när man blandar

dessa, eller utsätter dem för laserljus eller röntgenstrålning.

Egenskaperna hos en atom, en molekyl eller ett kluster bestäms av dess

geometriska struktur (hur atomer eller molekyler sitter ihop i en större en-

het) och av dess elektroniska struktur (hur elektronerna är fördelade kring en

atom, en molekyl eller i ett kluster). Vi har undersökt (med direkta metoder)

denna elektroniska struktur, och kan genom denna dra ganska långtgående

(indirekta) slutsatser också om den geometriska strukturen hos de studerade

systemen i vissa fall.

1.1 De olika experimenten

Grovt sett kan vi dela in denna avhandling i tre delar. De första tre inklud-

erade arbetena behandlar processer i molekyler, som sker när en molekyl be-

strålas med röntgenstrålning av varierande energi. I de två första arbetena har

vi undersökt hur molekylers beteende förändras då de sitter ihop med andra

molekyler av samma sort i ett kluster. Systemen vi har studerat är kluster av

tusentals syremolekyler och kluster av hundratals metylbromidmolekyler. Vi

har även studerat vad som händer när man bestrålar en mer exotisk molekyl,

nämligen natriumdimeren, Na2, utan att den befinner sig i ett kluster. Vi ville

ta reda på om molekylerna går sönder när de bestrålas, och i så fall om de går

sönder “lika mycket” och lika ofta när de befinner sig i kluster som när de

är i en gas. Att studera sådana system är intressant, eftersom det finns många

ställen där kluster och exotiska molekyler förekommer, men där man inte kan

15

komma åt att göra mätningar. Man tror till exempel att mycket av den absorp-

tion av den elektromagnetiska strålning som kommer från rymden görs av små

vattenkluster, och att mycket av den kemi som sker i atmosfären (till exempel

den s.k. Ozon-cykeln) katalyseras på olika sätt genom förekomsten av kluster.

Vi har också studerat den geometriska strukturen hos kluster av metylbro-

mid. Metylbromidmolekylen är vad man kallar en permanent dipol, vilket be-

tyder att många av elektronerna i molekylen finns vid en av dess ändar. Detta

gör att den får en elektriskt negativ laddning i änden där de flesta elektronerna

finns, och en positiv laddning där det finns färre elektroner. Detta fenomen

kallas för polarisation. Strukturen hos kluster som skapats genom att vi låtit

kluster krocka med med atomer och molekyler som då fastnat på eller i klus-

tret (så kallad “dopning”) har även den undersökts. Man kan tänka sig att detta

sker på ungefär samma sätt som om man skulle hälla russin på en stor klump

smör. Som figur 1.1 visar, så finns det två varianter av hur detta kan ske. Vilken

som inträffar beror på smörklumpens temperatur.

Figur 1.1: Den vänstra bilden visar vad som händer med russinen som hälls på smörsom man precis tagit ur kylskåpet. Den högra bilden visar vad som händer med russi-

nen om de hälls på smör som man värmt upp i en kastrull.

Om man precis har tagit ut smörklumpen ur kylskåpet så fastnar som vi alla

vet russinen på smörets yta, men om vi smält smöret innan vi häller russinen

på det så hamnar de inuti det som tidigare var smörklumpen. Ungefär samma

sak händer med klustren när de dopas, eftersom varje krock som klustret ut-

sätts för tillför det lite värme. Vi har mätt hur mängden kollisioner påverkar

strukturen hos dessa kluster.

Sist, men inte minst, har vi också mätt vad som händer med den elektroniska

strukturen hos två olika atomslag (natrium och kalium) som först belyses med

laserljus, och sedan bestrålas med röntgenstrålning. Sådana mätningar kan

ge värdefull information om hur den elektroniska strukturen i en atom eller

molekyl ser ut innan den belyses med laserljus, eftersom det inte alltid är helt

16

ökande våglängd ökande energi

joniserande strålningicke-joniserande strålning

Gamma

IR

Mikrovågor

Radio

Lågfrekvens UV

Synligt

Röntgen

Figur 1.2: Figuren visar olika våglängder av elektromagnetisk strålning, samt vad

dessa brukar kallas.

enkelt att studera detta på grund av diverse effekter som gör att elektronstruk-

turen blir “diffus”; somliga elektroniska tillstånd kan till exempel blanda sig

med varandra i atomens grundtillstånd, så kallad konfigurationsväxelverkan.

1.2 Experimentella metoder

En mängd kompletterande experimentella tekniker har använts för att genom-

föra de studier som presenteras i denna avhandling. De flesta av dessa kan

klassificeras som s.k. elektronspektroskopier, men även andra tekniker, så som

mass-spektroskopi och fluorescensmätningar har varit till hjälp under det ex-

perimentella arbetet.

1.2.1 Elektromagnetisk strålning

Egentligen är ljus och röntgenstrålning samma sak, och kallas tillsammans

med många andra former av strålning (ex. vis. radiovågor, gammastrålning

och mikrovågor) för elektromagnetisk strålning. Det som skiljer de två först-

nämnda åt är våglängden, alltså energi-innehållet hos var och en av dem. Rönt-

genstrålning innehåller mycket mer energi än vanligt ljus gör. Figur 1.2 visar

en bild av de olika våglängdsområdena för elektromagnetisk strålning.

I våra experiment används dels laserljus och dels röntgenstrålning för olika

ändamål. Laserljuset främsta användningsområde var att preparera exciterade

tillstånd i metallatomerna, medans röntgenstrålningens främsta uppgift var att

jonisera våra prov.

1.2.2 Elektronspektroskopi

Efter det att vi joniserat vårt prov med röntgenstrålning kan vi mäta en mängd

olika saker. En möjlighet är att mäta massan hos den kvarvarande jonen, för att

på så sätt få reda på om systemet vi joniserat har gått sönder i processen eller

17

ej. En annan möjlig mätning är att mäta energin hos den elektron som frigörs

från molekylen genom jonisationen. Det är detta, som kallas för fotoelektron-

spektroskopi, som vi oftast utför i våra experiment. Figur 1.3 visar en schema-

tisk bild över den elektroniska strukturen hos en atom. De energinivåer där det

finns, eller potentiellt kan finnas elektroner kallas för orbitaler.

E E

} Fylldavalensorbitaler

Tom valensorbital

Core-orbital

Grundtillstånd Exciterat tillstånd

Laserljus

Jon

isation

sgrän

s

Figur 1.3: En schematisk bild över elektronstrukturen hos en atom i sitt grundtillstånd,och i ett exciterat tillstånd.

Man kan även mäta så kallade Augerelektroner. Vid en röntgenjonisation

bildas en vakans i elektronstrukturen. Detta gör att elektroner i den yttre elek-

tronstrukturen känner en positiv laddning. Elektronerna kommer, eftersom de

själva har negativ laddning, attraheras mot denna vakans, och rusar dit för att

fylla den. Den energi som frigörs i en sådan process ges till en annan elektron

– Augerelektronen – som frigörs från systemet. Sluttillståndet i detta fall är ett

dubbelladdat tillstånd.

Det finns även en variation på samma tema, så kallad resonant Augerspek-

troskopi. Här föregås det s.k. Augersönderfallet inte av en jonisation, utan av

en excitation av en elektron till en tidigare tom elektronplats, som befinner

sig mycket långt ut i elektronstrukturen. Detta har ungefär samma effekt som

en jonisation, med skillnaden att den exciterade elektronen kan deltaga i det

påföljande Augersönderfallet. Man får alltså en jon med ett enkelladdat slut-

tillstånd. Resonant excitation försätter många molekyler i dissociativa till-

stånd, vilket innebär att molekylen går sönder. Detta kan ske mycket snabbt,

ofta inom ett fåtal femtosekunder. Om en molekyl går sönder så fort så hinner

inte Augerjonisationen hända innan molekylen har blivit till två fragment. Ef-

fekten av detta blir att man kan mäta Augersönderfallet från en av de två bitar

av molekylen som skapats, och på så sätt studera vilka fragment som bildas

och hur fort fragmenten fjärmar sig från varandra.

18

Vi använder även en teknik som kallas för röntgenabsorption, med vilken vi

studerar de tomma elektroniska orbitalerna hos atomer, molekyler och kluster.

1.3 Beräkningar

För att bättre förstå de saker vi ser när vi gör våra experiment använder vi

oss av datorberäkningar, med hjälp av vilka vi kan förutsäga både hur den

elektroniska och geometriska strukturen hos en molekyl eller i ett kluster ser

ut. Vi har använt oss av diverse beräkningsmodeller, dels av så kallade abinitio-beräkningar, som förutsäger elektronisk och geometrisk struktur genomkvantmekaniska beräkningar utan att några parametrar anpassas till experi-

ment. Ett kluster är ofta för stort för att kunna behandlas med kvantmekaniska

metoder. Vi använder därför även så kallade molekyldynamiksimuleringar.

I dessa beräkningar använder man en kombination av klassisk- och kvant-

mekanik för att förutsäga, till exempel, strukturen hos ett kluster.

1.4 Resultat

För de första tre arbetena som är inkluderade i denna avhandling har våra

experiment visat följande. Syremolekyler i kluster går fortfarande sönder i

samma omfattning som fria molekyler om de bestrålas med röntgenstrålning

av rätt energi. Metylbromidmolekyler i kluster, å andra sidan, verkar gå sönder

i mindre utsträckning när de bestrålas. Vi kan förklara detta genom att det i

metylbromidklustren verkar finnas vad som brukar kallas för “bandbildning”,

som är ett välkänt fenomen som förekommer i fasta material. För natrium-

molekylen har vi studerat hur avvikelsen från den atomära, sfäriska, geometrin

påverkar de energinivåer som uppkommer. Vi har också karaktäriserat den re-

pulsiva potential som uppstår då ett Augersönderfall sker i en sådan molekyl

som en Coulombpotential.

För de tre följande artiklarna har vi utfört experiment med syftet att stud-

era geometrin hos kluster av diverse atomer och molekyler. För kluster av

argon, dopade med syremolekyler, finner vi att syremolekylerna ligger kvar

på ytan fram tills dess att dopningsgraden (d.v.s. antalet kollisioner mellan

argonklustret och syremolekyler) är tillräckligt hög; då blir argonklustret fly-

tande och syremolekyler kan åka in i det (jfr. figur 1.1). En liknande situ-

ation uppkommer när vi studerar vad som händer då vi dopar argonkluster

med krypton eller xenon. I dessa två senare fall visar jämförelser med vad

som händer när man skapar blandade system genom så kallad “co-expansion”

(när man blandar de två gaserna och sen gör kluster av gasblandningen) att

dopningsproceduren tillåter skapandet av geometrier som är långt ifrån den

termodynamiska jämviktspunkten. Även detta är en indikation på att klustren,

vid låga dopningsgrader, inte har smält. Strukturen hos metylbromidkluster

19

studerades också. Vi kom fram till att strukturen hos dessa molekylära kluster

stämmer väl överens med den man hittar i den fasta fasen.

I de sista två artiklarna undersökte vi vad som sker när man joniserar atomer

som i initialtillståndet är exciterade. Vi använde ett lasersystem för att skapa

dessa exciterade tillstånd. En schematisk bild över hur detta går till kan ses i

figur 1.3. I den första av artiklarna kunde vi i natriumatomer konstatera att ex-

citeringen har som konsekvens att en magnetisk likriktning introduceras i sys-

temet. Vi kunde även observera en effekt som kallas för “shake-down”, ned-

skakning. I en sådan nedskakningsprocess inducerar jonisationen av atomen

en “skakning” i elektronstrukturen, som leder till att den i förväg exciterade

elektronen ger sin energi till den elektron som är på väg ut ur systemet. I och

med detta så ramlar den exciterade elektronen ner till det ställe den kom ifrån

från när den exciterades. Således får man i en nedskakningsprocess från en i

förväg exciterad atom samma elektroniska sluttillstånd som man får vid jon-

isation av en atom i sitt grundtillstånd, men med en elektron som kommer ut

från systemet med högre energi. Den extra energin motsvarar den energi som

tillförts atomen av laserljuset.

I den andra artikeln studerades kaliumatomer som också var exciterade med

hjälp av laserljus. Även här observerades nedskakning. Fenomenet utnyttjades

i detta fall till att karaktärisera de toppar som sågs i våra experimentella spek-

tra. Detta är möjligt, då nedskakningen, trots att sluttillståndet är detsamma för

den som för jonisation av en atom i sitt grundtillstånd, följer en annan sönder-

fallsväg. Symmetrin mellan de olika elektroniska tillstånden påverkar sanno-

likheten för vilka tillstånd som kommer populeras av elektroner. Eftersom den

exciterade elektronen kommer från en elektronorbital med annan symmetri än

den elektron vi senare mäter energin hos, kan vi direkt i våra experimentella

spektra se vilka toppar som är av en sådan karaktär att de blir tillåtna i och

med att vägen till sluttillståndet förändras.

20

2. Introduction

This thesis is based on investigations of the electronic and geometric struc-

ture of gas-phase clusters and metal vapors. Our method of choice is, primar-

ily, photoelectron spectroscopy. Used together with supplementary techniques

such as mass spectroscopy, and with the support of first principles calcula-

tions, this family of experimental methods allow for this and more. Common

to most of the techniques used in this work is that they rely on photons to

create electrons, which can then be measured.

Over the years, new light sources, like synchrotrons, lasers and free electron

lasers (FEL), together with extensive development in instrumentation for elec-

tron detection have made a large array of experiments possible, like studies of

magnetism, ultra-fast processes and charge- and nuclear dynamics. In this the-

sis, synchrotron radiation has been used to study ultrafast phenomena, com-

position of artificially created cluster structures, and fundamental properties

of certain exotic molecules. Lasers were used together with synchrotron radi-

ation in some of the experiments to, for example, prepare and probe aligned

samples and to probe processes occuring in the decay of excited states.

There are many motivations for performing this kind of research, both

within the area of fundamental research and within more practically applied

fields; like miniaturization and improvement of hard-drives, solar cells,

computer chips, exhaust catalysis, hydrogen fuel cells and so on. Gas

phase clusters, which has been the main focus of the work in this thesis

are important model systems, which are said to “bridge the gap” between

the single atom or molecule and the infinite solid [1]. They do so by

presenting an opportunity to study the size-regime where miniaturization of

transistors, magnetic storage etc. becomes influenced by quantum effects.

Figure 2.1 shows a schematic figure of a generic physical property as a

function of cluster size. As depicted in the figure, for small clusters, there are

discontinuous jumps of the property depicted when varying the size. For

larger clusters, the property as a function of size converges towards the bulk

value, and follows a more continuous trend. Exactly where the transition

between the “quantum” and the “bulk” regimes is varies for different

properties, and for different compounds. This research topic is highly active,

and attracts a lot of interest.

Likewise, metal vapors, which have been the second subject of study in

the thesis, present a way of studying the individual properties of the kind of

atoms that form many of the most common solids that are used in everyday

21

life. This leads to a better understanding of how to model solids from existing

knowledge about atomic and molecular properties.

Quantum regime

Bulk regime

Bulk value

Pro

per

tyP

(N)

Size N∞1

Figure 2.1: Schematic overview of a generic property versus cluster size. In the bulkregion, the value P(N) follows a continuous trend towards an asymptote, while in the

quantum region there are significant variations between the various sizes.

Another very appealing property of clusters is that their surface area is very

large, i.e. that almost all atoms or molecules are situated on their surfaces. Ina solid, an infinitesimal percentage of the atoms are situated on the surface,

while in a cluster of 13 atoms 92% are found in the surface layer. Even in

a cluster of a size of several thousands of atoms, � 25% of the constituent

particles are found in the surface layer. This means that, in contrast to the case

of the solid, surface effects considerably affect physical properties such as the

melting point, conductivity, ionization potential and absorption frequencies in

clusters [2, 3].

The cluster part of this thesis deals with rare-gas clusters, molecular clusters

of O2 and CH3Br, and mixed rare-gas/molecular clusters. All of these systems

form clusters mainly by van der Waals interaction. The metal vapor part fo-

cuses on alkali-metals, namely sodium and rubidium, and dimers of sodium.

The information provided by studies of this kind of systems can provide in-

sight into matters not accessible to solid-state experiments.

The approach of this thesis is to begin with a brief overview of some fun-

damental concepts, such as electronic and geometric structure of molecular

and cluster objects, to describe the experimental techniques used to create and

probe such systems, and then to give a more detailed description and discus-

sion of the experiments performed in the papers included in the thesis.

22

3. Fundamental concepts

To facilitate comprehension of the works that are presented in this thesis, the

following overview of various fundamental aspects of related subjects such

as the electronic structure of atoms, molecules and clusters, the geometrical

structure of molecules and clusters and the photoelectric principle will be dis-

cussed briefly in the following chapter.

3.1 The electronic structure of matter

What we commonly think of as ’matter’ consists of large collections of atoms

or molecules. An atom is composed of a nucleus and of electrons. The nucleusconsists of protons and neutrons. In neutral ground state cases, the number of

electrons in an atom is equal to the number of protons in the nucleus. The

number of neutrons can vary, and gives rise to various isotopes of the same

atom. Molecules consist of several atoms bound together by intra-molecular

forces. How the electrons are arranged in the atoms or molecules determine

how these particles interact with each other within the lump of matter, like if

they form a solid iron ingot, or if they form a puddle of water. The arrange-

ment of electrons also determines how the matter interacts with other nearby

pieces of matter through, for example, magnetism. To understand such macro-

scopic properties of large collections of matter, a good understanding of the

electronic structure on the atomic and molecular level is needed.

3.1.1 Atoms

An atom is the most basic system that has an electronic structure. It is from the

understanding of atomic electron structure that molecular electron structure is

inferred. The electrons are arranged in a somewhat peculiar manner, following

what is known as the Aufbau principle, as a consequence of the quantization ofenergy [4]. The most basic structure in the electron arrangement is the shell.This is usually denoted either by a capital letter;K,L,M, ... or by a correspond-ing principal quantum number n = 1,2, ... In this thesis, the number notationwill be used throughout. Electrons in a shell with a lower quantum number

are, typically, more tightly bound to the nucleus, and generally contribute less

to the chemical properties of a compound than electrons in a shell with higher

quantum number.

23

A shell can also be divided into subshells. In this case, what differentiatesthe electronic states from each other are their orbial angular momentum. Con-

ventionally, the subshells are denoted by lower case letters, s, p,d, f , ..., corre-sponding to angular momentum quantum numbers l = 0,1,2, . . . ,(n− 1). Asin the case of the shells, electrons with a lower l quantum number are gener-ally more tightly bound to the nucleus than those with higher ones. The shell

and subshell can be combined into an orbital, which is designated by nl, forexample 2s. In this example, the orbital describes the electrons in shell n = 2

and subshell l = 0.

In many cases, this is enough information to specify the complete elec-

tronic structure of an atom, due to the fact that various constraints such as

the Pauli exclusion principle [5] implicitly determine the remaining quantum

numbers. Such a description is done by utilizing an atomic configuration. Aconfiguration is composed of one or more orbitals, and might look like (1s);corresponding to the H atom in its ground state, where there is only one elec-

tron, which is located in the first shell, and in the first subshell of that shell,

(1s)2; corresponding to the He atom, also in its ground state, which has twoelectrons in the first shell, and in the first subshell. A sodium atom, which has

11 electrons, has the configuration (1s)2(2s)2(2p)6(3s) in the ground state.

s-orbital

px-orbital py-orbital pz-orbital

x

xxx

y

yyy

z

zzz

Figure 3.1: Schematic picture of contour surfaces for s and p orbitals.

One designates the projection of the electron orbital angular momentum�lonto the z-axis of the reference system as ml , the magnetic quantum number.

The definition is such that lz = ml · h. This number is restricted to the values

24

ml = 0,±1,±2, ...,±l. As a consequence, there is only one energy level forelectrons in s-subshells (ml = 0), while there are three for p-subshells (ml =0,ml = ±1). In the case of the p-subshell, ml = 0 corresponds to the orbital

along the z-axis, pz, and ml = ±1 corresponds to the orbitals along the x- andy-axes, px and py. When visualizing orbitals, contour surfaces are normally

used, where the surface is taken to be at a distance from the nucleus within

which there is a 90% chance of finding the electron at any given moment.

An example of s and p contour surfaces is shown in figure 3.1. Each electronalso carries what is known as spin. The spin quantum number for an electron iss = 1/2. Analogue to the case of the angular momentum vector of the electron,the projection of the spin vector �s along the z-axis is known as ms, and can

take values ms = ±1/2. Typically, one also defines a total angular momentum�j =�l +�s, and its projection along the z-axis is designated m j = ml +ms.

In many-electron systems, one uses the vector sum of the individual elec-

tron angularmomentum vectors; �L = ∑i�li. This angular momentum is quan-

tized by an integer quantum number L such that |L|= h ·√L(L+1). A similarapproach is taken when describing the spin of such systems. The quantum

numbers ML = 0,±1,±2, . . . ,±L and MS = 0,±1,±2, . . . ,±S are the projec-tions of �L and �S along the z-axis of the reference systems also in this case.In addition, ML = ∑ml and MS = ∑ms. The orbital angular momentum and

spin are connected also in this case by the total angular momentum vec-

tor �J. The total angular quantum number J is calculated according to J =L+S,L+S−1, . . . , |L−S| and the projection value MJ = 0,±1,±2, . . . ,±J.In the cases where additional information is needed, such as about the spin

of the electrons, the configuration is usually accompanied by a term symbol.The term symbol is constructed by using the quantum numbers L, S and J, andis taken to be 2S+1LJ . Similar to the simple case of the subshells, the quantum

number L = 0,1,2, ... is designated S,P,D, .... The term symbol notation is

very useful when describing, for example, excited or ionic states. A distinction

is usually made between the more tightly bound electrons in an atom, and

those electrons that are easier to remove. The electrons that are tightly bound

to the nucleus are called core electrons. The remaining electrons, which arenot as strongly bound, are known as valence electrons, and are the electronsprimarily responsible for forming bonds with other atoms. More specifics can

be found in a regular text-book, such as [6].

3.1.2 Molecules

A molecule is commonly defined as being a stable, electrically neutral group

of atoms, held together by chemical bonds [7]. In this thesis, molecules con-

sisting of atoms bound by the sharing of electron pairs have been studied.

Such bonding is known as covalent bonding. Examples of such covalently

bound systems are the O2, H2O and Na2 molecules. Many of the terms used

to describe the electron structure of molecules are conceptually similar to

25

those used in the atomic case, and most understanding of molecular electron

structure stems from using various schemes of combining atomic orbitals into

molecular orbitals.

A way to describe the electron structure of a molecule is by using an

approach known as molecular orbital linear combination of atomic orbitals(MO-LCAO). As is implied by the name, this approach makes use of the

sums of atomic orbitals to combine them into molecular orbitals. In, for

example, a diatomic molecule, the spherical symmetry of the electrostatic

field of the individual atom is broken, and replaced by a cylindrically

symmetric field. For electrons in the molecule, the angular quantum

momentum vector �l precesses around the symmetry axis of this field, witha constant value ml = l, l − 1, l − 2, ...,−l. In molecules, the orbital angularquantum number l is designated by λ , and analogous to the atomic casesubshells λ = 0,1,2, ... is designated as σ ,π,δ , .... Molecular orbitals aredesignated by their principal quantum number and their angular momentum

quantum number. Orbitals that are rotationally symmetric around the z-axisare σ -orbitals. Thus the atomic s- and pz-orbitals both become σ -orbitals in adiatomic molecule. The px- and py-orbitals become π orbitals, and so on.If we take the example of O2, we can construct the molecular electronic

configuration of the molecule by using the electronic configuration for indi-

vidual O atoms. Each such atom has 8 electrons, in a (1s)2(2s)2(2p)4 config-uration. A schematic picture of how the combination can be done is shown in

figure 3.2.

The configuration of the O2 molecule becomes

(1σ)2(1σ∗)2(2σ)2(2σ∗)2(3π)4(3σ)2(3π∗)2

where the ∗ denotes antibonding orbitals. Term symbols are used also in the

case of molecular electron structure, to describe spin and geometry. One then

uses quantum numbers corresponding to the sum of the individual electron

spins and orbital angular momenta. The individual electron spin projection

quantum number, which in the atomic case is denoted ms is denoted σ . Thisshould not be confused with the λ = 0 quantum number designation σ . Thetotal angular momentum is denoted by ω and is defined as ω = λ + σ . Thesums are designated by Λ, Σ andΩ. Note that a simple arithmetic sum is suffi-cient in the case of molecules, since both λ and σ are, inherently, projectionson the molecular axis. The term is given by 2Σ+1ΛΩ. Sometimes, the mirror

symmetry of the spatial part of the electron wavefunction is given by adding

either a + or a - to the term symbol, and sometimes the property of inversion

symmetry for the entire wavefunction is also denoted by a g or u (gerade orungerade) added to the term. The oxygen molecule, for example, is a 3Σ−

g in

its ground state. For an in depth discussion, see for example [4].

26

1s1s

2s2s

2p2p

1σ

1σ∗

2σ

2σ∗

3σ

3σ∗

3π

3π∗

E

OO O2

Figure 3.2: Schematic picture of the electron structure of O2 The atomic levels thatcontribute to certain molecular levels have a dashed line connection to these levels.

The ∗ denotes antibonding orbitals.

3.1.3 Clusters and solids

The step towards bulk solids from atoms or molecules is rather large. In be-

tween the two exists a category of objects known as clusters [1]. A clustercan be loosely defined as a collection of atoms or molecules held together

by intra-molecular forces, such as van der Waals forces, polar forces, metal

bonds etc. Some objects in this category can be classified as both molecules

and clusters, and the nomenclature is not always entirely unambiguous. Com-

mon to all clusters are that they consist of 3 to some 107 atoms or molecules.

The electronic structure of clusters evolve very differently depending on the

type of bonding mechanism(s) present in the cluster. This thesis focuses on

van der Waals clusters, and in these, the electronic structure of the consituent

atoms or molecules is, in most cases, largely unperturbed from the gas phase

case. There are some differences, where atomic and molecular orbitals are

compressed by neighbours in the cluster [8], and there might be breaking of

molecular symmetries in the cluster lattice to some extent [9], but in the ex-

27

periments performed within the framework of this thesis, these differences

cannot be made out due to limitations in the resolution.

When a cluster becomes large enough, it becomes a bulk solid. It has then

acquired all solid state properties, like conductivity, magnetism and heat ca-

pacity. The electron structure of solids is characterized by electron bands,

which are, in principle, overlapping orbitals in which electrons can move

freely in the entire solid. As this is beyond the scope of this thesis, a further

description will not be given here. For a detailed discussion, see for exam-

ple [10]

3.2 The geometric structure of matter

In addition to the electronic structure, matter also possesses geometric struc-

ture. For atoms, one can define certain radii, beyond which interaction with

other atoms is neglible, such as the covalent radius and the van der Waals ra-

dius [11]. These determine at which distances the atoms bind to other atoms

with the respective kind of bonding mechanism. These, together with the elec-

tronic structure determine how the atoms form molecules, clusters and solids.

Molecules are, as previously mentioned, composed of several atoms. This

means that they can be classified within certain geometrical point groups [12].Knowing what point group a molecule belongs to gives information about

the geometry of the electronic orbitals, about the vibrational modes of the

molecule and about which transitions between the different orbitals that are

possible. In this work, molecules of three kinds have been studied. These kinds

are homo-nuclear, diatomic molecules belonging to the point group D∞h, such

as O2 and Na2, and the more complex CH3Br molecule, belonging to the point

group C3v.

As a part of the description of the different electronic and geometric states

of a molecule, potential surfaces are often used. These surfaces map the energy

of the molecule as a function of the internuclear distances, and are often used

to to predict molecular dissociation limits, equilibrium distances in molecules

and to interpret vibrationally resolved spectra. The use of potential surfaces is

motivated by the Born-Oppenheimer approximation, which states that electronand nuclear movements can be decoupled [13]. This is not entirely true, but

in most cases it gives a good agreement to experiments. Figure 3.3 shows

an example of a 1-D potential surface (i.e. a potential curve) for the oxygen

molecule in its ground- and first ionized state.

The horizontal solid lines in the potential wells in figure 3.3 mark the vibra-

tional levels of the oxygen molecules in its different states. Vibrations come

into play for molecules, since they consist of more than one nuclei, which

can move with respect to each other. These intramolecular vibrations can, for

example, be induced by absorption of, or ionization by radiation. A change

in the vibrational state of the molecule is governed by the transition moment

28

Internuclear distance (A)

Pot

ential

ener

gy(a

rb.

unit

s)

X 2Πg

X 3Σ+

g

0 1 2 3 4 5

FC region

Figure 3.3: The potential surfaces of the oxygen molecule in its ground state (bottom)and in its first ionized state (top). The Franck-Condon region is marked with a shaded

rectangle, the horizontal solid lines are some of the vibrational levels.

.

�Rv. The intensity of a given vibrational transition is proportional to the square

of the transition moment, i.e. �Rv2. In a notation where �μ is the dipole opera-

tor, the first vibrational level is denoted by Ψi and the second one by Ψ f , the

transition moment is given by the integral

�Rv =∫

Ψi�μΨ f dR (3.1)

The transition moment depends somewhat on the internuclear distance in

the molecule. If this dependence is disregarded, one arrives at the Franck-Condon (FC) approximation. The approximation states that the transitions be-tween two vibrational levels can be treated as vertical in a potential diagram

like the one in figure 3.3. The square of the overlap integral between the initial

and final vibrational state

F=(∫

ΨiΨ f dR)2

(3.2)

if known as the Franck-Condon factor. Vibrational transitions can be said to

be proportional to this factor, since the equilibrium distance in the molecule,

29

Re, is constant within the FC approximation. The shaded area in figure 3.3

is the so-called Franck-Condon region. Most of the vibrational transitions

upon excitation or ionization takes place within this region. Knowing the FC-

region, and by measuring the relative intensities of the different vibrational

levels, it becomes possible to experimentally determine potential surfaces of

molecules. This is highly desirable, as they can provide information about

equilibrium distances, vibrational frequencies, dissociation limits and a vari-

ety of other molecular properties.

Clusters can be formed in a large variety of geometries, depending on the

thermodynamical conditions during the formation process, on the method of

production and on the type of bonding mechanisms present in the clusters.

Typically, the geometrical structure of clusters resemble the geometrical struc-

ture of the infinite solid more and more as the cluster grows in size. In this

thesis, the radial segregation of oxygen molecules in large Ar clusters have

been investigated.

The infinite solid is characterized by its crystal structure or lack thereof. As

in the case of molecules, one can classify various kind of geometries within

mathematical groups, which determine various properties. This is a subject

discussed in for example [10].

3.3 The photoelectric effect

When light shines onto matter, be it a single atom or a sheet of metal,

electrons can be ejected. This is provided that the light is of sufficient energy

to overcome the so-called work function, Φ of the material, or ionization

threshold in an atom or molecule. This effect was discovered by H. R. Herz in

1887 [14], and explained by A. Einstein in the famous paper from 1905 [15].

A schematic picture of the effect is shown in figure 3.4.

What Hertz found experimentally, and Einstein explained using the quan-

tization of energy, is that electrons are emitted from a material with a kinetic

energy related to the energy of the incident radiation. In principle

Ekin = hν −Φ−|EB| (3.3)

describes the relation. Ekin is the kinetic energy of the emitted photoelec-

tron and EB is the binding energy of the electron. The work function Φ is

well known for many materials. All energies and calibration of spectra in this

work are given with respect to the vacuum level, as is commonly done when

working with atoms, molecules and clusters [16].

The photoelectric principle is the corner-stone of photoelectronspectroscopy (PES), the family of experimental techniques employed in thiswork, since it allows for the determination of the binding energy of electrons

30

Solid Single atom

Figure 3.4: A schematic picture of photoelectrons being emitted from amaterial underirradiation. The leftmost image shows a solid piece of material filled with an electron

gas, and the rightmost shows an atom with well defined energy levels for the electrons.

by measuring their kinetic energy. These various techniques are further

described in a following chapter.

31

4. Experimental equipment

The experimental work in this thesis was performed mostly at the MAX lab-

oratory in Lund, Sweden, and at the department of physical sciences at Oulu

university, Oulu, Finland. The following chapter gives an introduction to the

experimental equipment used in the experiments. The concepts of synchrotron

radiation and laser light are introduced, along with a description of the exper-

imental systems and the way they are prepared and handled. The MAX labo-

ratory and the beam-line I411 will be presented more thoroughly, since this is

were most of the research was done.

4.1 Vacuum

The different kinds of experiments, such as photoelectron spectroscopy, per-

formed in this thesis have high demands for vacuum around the sample. This

is due to the fact that the spectroscopic information in such an experiment is

mediated by electrons, which interact strongly with air. In a vacuum, the elec-

trons are free to fly a long distance without interacting with any other particles

and thus to preserve the spectroscopic information about the probed sample.

Similarly, a synchrotron storage ring requires even higher vacuum, since the

electron beam that is stored in the ring would be quickly depleted and scat-

tered at atmospheric pressure, and since the soft x-rays emitted from it would

be absorbed quickly in an atmosphere.

Typically, the pressure at the experimental station should be at most in the

10−5 mbar range for electron spectroscopy to be feasible. In the synchrotronring, pressures in the 10−10 mbar range are common. To maintain this kind ofvacuum is quite hard, and requires continuous pumping of the experimental

chamber and surrounding equipment [17]. In our experiment, we use standard

oil roughing pumps, and turbomolecular pumps of different sizes to maintain

vacuum even under very demanding experimental conditions (i.e. very high

gas loads). An additional benefit of performing experiments in vacuum is that

the sample contamination is inherently low, since there are fewer particles in

the surroundings that can interact with the sample before it is probed. This

also has the consequence that there will be less residual contributions to the

experimental spectra acquired under vacuum conditions.

33

4.2 Synchrotron radiation

In 1947, Elder, Gurewitsch, Langmuir and Pollock, at the General Electric

synchrotron accelerator, observed light emitted from their machine [18]. First

thought to be Cherenkov radiation, it soon became apparent that what was

seen was something else. This radiation turned out to be produced by the ac-

celerated electrons that were travelling at relativistic speeds in the synchrotron

accelerator, and thus it was termed synchrotron radiation. The explanation was

given by Schwinger in 1949 [19]. Some years later, in 1956, synchrotron ra-

diation was also observed in astronomical processes by Burbridge [20]. In the

case of astronomical processes, high-energy electrons are accelerated in the

magnetic fields of cosmic objects, producing jets of synchrotron radiation.

The development of synchrotron light sources have gone on from the hum-

ble beginnings in the General Electric lab, to the large 3rd generation syn-

chrotron facilities in operation at a multitude of places today. Development of

4th generation facilities and the so-called x-ray free-electron lasers (FEL) is

ongoing, with the first such facilities just recently becoming operational.

In a 3rd generation synchrotron facility, the most important part is the elec-

tron storage ring, where electrons, as the name implies, are stored in a cir-

cular orbit at relativistic speeds. The storage ring consists of long, evacuated

tubes, magnetic structures and conditioning devices to control the electron

speed, beam-path and divergence. The velocity of the electrons determines

the amount of beaming, i.e. the brightness of the light, in such a way that the

closer to light speed the electrons travel, the more light is emitted into the for-

ward tangential direction of the electron velocity [21]. This is schematically

shown in figure 4.1.

�v�v

�a�a

a) b)

Figure 4.1: The emitted power at two different electron velocities. In a), β = 0 and

in b), β = 0.9. The emission is beamed in the tangential direction of the electron pathwhen at relativistic speeds.�a and�v denotes acceleration and velocity, respectively.

34

At certain points in the magnetic structure, synchrotron radiation can be ex-

tracted in a number of ways [22]. Either, by using one of the ring bending

magnets, which produces a rather broadband and comparatively low-intensity

radiation, or by introducing a so-called insertion device into the ring. An inser-

tion device can, for example, be an undulator or a wiggler. In these, periodic

magnetic structures are used to extract synchrotron radiation. The characteris-

tics of wiggler- and undulator radiation differ from bending magnet radiation,

and from each other, in the intensity and the wave-length distribution of the

generated radiation. Figure 4.2 shows a schematic picture of the three afore-

mentioned magnetic structures.

e−

e−

hν

hν

Bending magnet Undulator/Wiggler

Figure 4.2: Schematic picture of a bending magnet, and an insertion device of theundulator/wiggler type.

Common to all of these kinds of artificially generated synchrotron radiation

is that they all have some very peculiar characteristics, namely:

• High brigthness, intensity and brilliance.• Tunability over a large energy range, from eV to MeV.• Well defined polarization.• Time structure.

35

For the end-users, this opens up possibilities to perform a wide range of exper-

iments that have hitherto been impossible, with regards to different techniques

and also for very dilute samples. For example, core level photoelectron spec-

troscopy, x-ray absorption spectroscopy and x-ray crystallography are tech-

niques that can be routinely applied to systems such as gas phase samples,

surface films, and protein crystals by utilizing synchrotron radiation.

4.3 Laser Light

Building on the fundaments laid down by Einstein [23] and other important

theoretical works by many prominent researchers, C. H. Townes demonstrated

the first working maser (Microwave Amplification by Stimulated Emission)

in 1953 [24], This device had a non-continuous output of microwaves. The

term “Laser” was coined by R. G. Gould in 1959, in a conference paper titled

The LASER, Light Amplification by Stimulated Emission of Radiation [25]. Asthe name suggests, a laser is a device that emits amplified light, rather than

microwaves.

Typically, a laser consists of two main parts; a gain medium and a oscilla-

tor cavity. The gain medium can be, for example, a crystal, a liquid, or a gas

mixture which is selected to provide optimal characteristics for some experi-

mental parameter, such as light wavelength or intensity. The oscillator cavity

is, in most cases, limited by two mirrors, one of which is semi-transparent,

thus letting a part of the amplified light through. Other designs might include

only one, or no mirrors, but they are limited by large beam divergence. Fig-

ure 4.3 shows a schematic picture of the difference in divergence between

zero, one and two mirror lasers.

a)

b)

c)

Figure 4.3: Three different laser designs. a) has no mirrors, and a large divergence, b)has one mirror and medium divergence, c) has two mirrors and small divergence.

36

Like synchrotron radiation, laser light has some very special properties that

makes it useful to perform a wide range of experiments [26, 27]. Among these

are the facts that lasers are

• Highly collimated.• Spatially coherent.• Quasimonochromatic, i. e. has a very narrow wavelength range.• Very bright, or have a very large radiant power.• Time structure.This makes them highly usable in various kinds of applications, in research

(spectroscopies, quantum optics etc.), in industry (drilling, cutting and so

forth) and in every day life (cd-players, bar code readers and many other com-

mon apparatus).

4.4 The MAX laboratory and beam-line I411

The MAX laboratory in Lund, Sweden, is one of two national research lab-

oratories, the other being the radio telescope facility in Onsala. At the MAX

laboratory, research in a wide range of topics is performed, utilizing the three

synchrotron storage rings and the linear accelerator at the facility. The three

storage rings are the MAX I, a second generation storage ring, the MAX II, a

third generation ring and the MAX III, also a third generation ring. Most of the

work in this thesis was performed at the I411 soft x-ray beamline at MAX-II,

where photon energies of roughly 60-600 eV are accessible. Figure 4.5 shows

an overview of the MAX laboratory.

Figure 4.4: The different elements of the beam-line I411 at the MAX laboratory inLund. The leftmost part is connected to the undulator insertion device, experiments

are performed at one of the experimental stations at the right end of the beamline.

This beamline and its experimental station is specially designed to accomo-

date high pressure experiments, like gas phase samples. This is achieved by

using many vacuum pumps and so-called differential pumping stages, which

provide a steep gradient in the local pressure in the beamline. In the experi-

mental station, pressures can be in the 10−5 mbar range without affecting thevacuum in the synchrotron storage ring measurably. The permanent experi-

37

Figure 4.5: The layout of the MAX laboratory facility in Lund, Sweden.

mental station of the beamline is equipped with a Scienta R4000 photoelec-

tron spectrometer. Just before the permanent experimental station is a part of

the beamline which can be interchanged with user experiments. It is in this

part of the beamline all of the laser experiments have been performed, using

a set-up built in Oulu, Finland [28]. The spectrometer in this case was a Sci-

enta SES-100 hemispherical analyzer. A drawing of the beam-line is shown in

figure 4.4

Beside the beamline is a sealed hutch, where a state-of-the-art laser system

is installed. This system consists of a 10W CW (continuous wave) Coherent

Verdi laser, operating at 532 nm wavelength, most often used to pump one of

the other lasers that are available. These other lasers are a Coherent 899-21

ring laser, which can use either a Ti:Sa crystal or a dye jet as gain medium,

providing an effective output power of up to 2 W CW in a wavelength range

of 550-1000 nm. There is also a pulsed Ti:Sa fs laser, giving optimal output at

around 600 nm wavelength. In addition to this, the equipment includes com-

puterized control of the cavity optics for the Coherent 899-21 laser, which

allows for wavelength scans, and a doubler ring allowing users to reach UV

wavelengths. A schematic picture of the CW laser system which has been used

in the present work is shown in figure 4.6.

38

Beam diagnostics

Referencechamber

Frequencydoubler

Verdi pump

Ti:Sa ringlaser

PolarizerFresnel rombλ/4I411 synchrotron radiation Experiment

Figure 4.6: A schematic overview of the different components in the CW laser system

employed in this work.

4.5 Sample production

In this thesis, beams of clusters of atoms and molecules as well as free metal

atoms were used as samples. There is no way to commercially obtain a bottle

of clusters or vaporized metal, so the samples have to be produced in situ. Thesections below will detail how this is done with our different set-ups.

4.5.1 Cluster production

Clusters can be produced in many ways, each with advantages and drawbacks

for certain types of experiments. In our experiments we are limited by the

photon flux at the undulator beam-line we use, which means that we have to

have a relatively high target density to be able to use electron spectroscopy as

an efficient experimental probe. Our method of choice is to produce clusters

from gases through adiabatic expansion, which produces fairly large clusters

in a large abundance. We have built our source in-house, and the set-up is

similar to that of [29].

The source works by feeding gas under high pressure (usually 1-5 bar) into

a stagnation volume, from which the gas is fed through a small, conical noz-

zle to an expansion volume. The pressure in the expansion volume may vary

between 10−5 and 10−3 mbar. The small conical nozzle has an opening of 150μm, and an opening angle of 20◦. Gas coming from the stagnation volume tothe expansion volume through the nozzle is adiabatically expanded, and forms

a supersonic jet [30] in the expansion volume. The clusters are mostly formed

on the axis of the jet, which means that it is possible to suppress the atomic or

molecular residue by introducing a skimmer in front of the nozzle. This skim-

mer also works as a separating stage in the differential pumping of the set-up.

The clusters and the residual atomic or molecular gas that pass through the

skimmer then enters the interaction region, where the synchrotron beam in-

tersects the cluster beam. Clusters formed in the jet-expansion are distributed

around a mean size 〈N〉, which is governed by the expansion parameters; noz-zle temperature, nozzle geometry and stagnation pressure [29, 31, 32]. Higher

pressure and lower temperatures generally give larger clusters.

39

Figure 4.7: Schematic picture of the cluster source (to the left) attached to the experi-mental station (to the right).

Our set-up is designed to allow for high stagnation pressures, liquid nitro-

gen cooling of the nozzle and relatively large nozzle diameters. Two large

turbo-pumps are used to maintain the pressure in the expansion volume. All

these considerations allow for a source that produces a dense enough beam to

perform even very demanding types of electron spectroscopies [33]. We use

calibrated pressure regulators and gauges to control the pressure in the experi-

ment, and a LN2 cooling system together with an electrical heater to maintain

stable nozzle temperatures down to around -170◦C. To give some idea of thecharacteristics of the source, it allows us to perform electron spectroscopy

experiments on Ar clusters in the size range from 100 to 50000 atoms. A

schematic picture of the cluster source is shown in figure 4.7.

4.5.2 Doped cluster production

To create structures of mixed clusters, we have used the post-expansion dop-

ing technique, pioneered by Scoles et al. [34, 35]. In our case, the doping stageconsisted of four needles of 150 μm inner diameter, mounted perpendicularlywith respect to the cluster beam and at a variable distance from the expansion

nozzle. The doping stage in this set-up is mounted before the cluster beam

is skimmed. This design minimizes the deflection of the cluster beam, which

passes through the centre point of the needle “cross”. A schematic picture of

the set-up is shown in figure 4.8. The doping stage is capable of delivering

most kinds of gaseous substances to the cluster beam, thereby allowing cre-

ation of, for example, a chemically reactive sample system.

40

Figure 4.8: Schematic picture of the doping stage. The four needles are mounted per-pendicularly with respect to each other, and with respect to the sample beam that

passes between them.

4.5.3 Metal vapor production

Metal vapors can, just like clusters, be produced in many ways. These differ in

the vapor density they provide and are suited to different applications. In our

experiment, we have chosen to use resistively heated ovens, which give a large

amount of metal vapour. Our experimental set-up with an oven was situated in

a user-endstation built in Oulu [28], which included a Scienta SES-100 elec-

tron energy analyzer. Another similar oven was situated inside the laser hutch

beside the beamline, to provide a sample for the reference chamber. Such a

reference chamber allows for finding laser flourescence lines and thus tuning

the laser system independently of what is going on in the main experimen-

tal chamber. Figure 4.9 shows the flourescence from 3s → 3p excited sodiumatoms, produced with this set-up.

41

Figure 4.9: Photograph of flourescense from laser excited sodium atoms.

42

5. Probing techniques

There are several different techniques within the broad definition of electron

spectroscopy. Those that have been used in this work will be given a thorough

introduction in the following chapter, together with a brief explanation of how

such spectra can be interpreted with the help of, for example, calculations and

curve fitting.

5.1 Ultra-violet Photoelectron Spectroscopy (UPS)

UPS, also known as valence photoelectron spectroscopy [36], is used to probe

the shallower part of the electron cloud, the valence electrons. The way the

photoelectrons are produced is by valence photoionization, a straightforward

photon in–valence electron out process. Figure 5.1a shows a schematic picture

of UPS. For the studies in this thesis, UPS has mainly been used as a tool for

roughly determining the sizes of clusters, but also, in the case of (Ar)m(O2)nclusters, to determine the structure of mixed cluster systems. Due to several

factors contributing to an efficient production of photoelectrons in this energy

region (such as the maximum photon flux of the beam-line I411 and the high

ionization cross-sections of most valence orbitals around 60 eV) it is also an

excellent tool for aligning the cluster beam with the synchrotron radiation

beam, and a good way to see contaminations in the spectra. Two examples of

UPS spectra can be seen in figure 5.2 where the leftmost spectrum is the UPS

spectrum of Argon clusters and the rightmost spectrum is the UPS spectrum

of O2 clusters.

One can note that the cluster feature in the Argon spectrum is very broad

compared to the atomic feature in the same spectrum. This is commonly at-

tributed to band formation of the valence states in rare-gas clusters [37]. In

the case of O2, on the other hand, the width of the cluster feature envelope is

not that much different from that of the molecular ditto. This effect is, so far,

not well studied, but might be due to the fact that the oxygen-oxygen symme-

try inside a cluster lattice prevents band formation or that the oxygen valence

orbitals are smaller than those of rare-gas atoms. UPS energies can be found

easily in the literature, for example in [38].

43

EE a) b)

γ

γ

Figure 5.1: a): Schematic picture of UPS. b): Schematic picture of XPS. Note thedifference in origin of the photoelectrons.

-1-1 00Relative binding energy (eV)

Inte

nsity

(arb

.un

its)

hν=61 eVhν=61 eV

Figure 5.2: Spectra of the 3p level of Ar clusters and of the X-state in O2 clusters.The dashed lines represent the total envelope in the cluster spectrum, the solid lines

represent the molecular envelope and the shaded areas represent, roughly, the cluster

features in the spectra.

5.2 X-ray Photoelectron Spectroscopy (XPS)

XPS, or core electron spectroscopy, is used to probe the electronic structure

deeper in the electron cloud than UPS. Just like UPS, it is a straightforward

photon in–electron out process, but where in UPS a valence electron is emit-

ted, the XPS electron is emitted from one of the core orbitals of the atom or

molecule. As mentioned in section 3, core electrons does not participate in

chemical bonding, and remain atomic-like even in a solid. Although this is the

case, there is a feature in XPS which makes is an extremely powerful tech-

nique for identifying various compounds. This feature is the chemical shift

of core level binding energies. This concept of chemical shift means that the

44

energy of a core level will be affected by its surroundings, which makes it

possible to identify and study even individual atoms of a certain type in, for

example, an alkane chain, since the atoms at different places in the molecule

will have different surroundings. The principle of XPS is shown in figure 5.1b,

and the XPS spectrum of O2 is shown in figure 5.3. In the case of O2, there

are two peaks in the XPS spectrum. These two components are due to the

paramagnetic splitting in the O2 molecule, which comes about because the

oxygen molecule has only two electrons in its highest occupied molecular or-

bital (HOMO), and because this HOMO has space for two more electrons.

Thus, O2 is said to be an open shell molecule, leading to this kind of splitting.XPS energies for a wide variety of species can easily be found in tables such

as for example [39].

542543544545546547548

Inte

nsity

(arb

.un

its)

Binding Energy (eV)

ClusterMolecularfeatures features

Figure 5.3: The O1s XPS spectrum of O2 clusters. Note that for clarity, the spectrumdisplays only the adiabatic vibrational components as shaded areas, while the solid

black line takes into account all of the vibrational components used in the fit. The

ionizing photon energy was 570 eV.

5.3 Auger Electron Spectroscopy (AES)

Auger electron spectroscopy is different from UPS and XPS in that it isn’t

merely a photon in–electron out process. Instead, AES studies the electrons

that are emitted when a core-hole decays. A schematic picture of this is shown

in figure 5.4. To observe Auger electrons, a core hole is first created by, for

instance, photoionization. The core-ionized state is unstable, and after a short

period of time (typically in the fs range) it will decay to a lower, more sta-

45

ble energy state. This may for instance happen by a valence electron “falling

down” into the core-hole, while another valence electron is emitted with a

certain energy. This latter electron is called an Auger electron and its kinetic

energy can be measured by an electron spectrometer. The bandwidth of the

ionizing radiation is of no consequence for the resolution in the Auger spectra,

since the kinetic energy of the Auger electrons is given by the energy differ-

ence between the energy levels involved in the Auger decay and the second

ionization threshold energy. Rather, the width of Auger lines will be deter-

mined by the lifetimes of the intermediate and final states. Therefore, features

in atomic Auger spectra may often be quite sharp. Auger electron energies can

give valuable information of the relative positions of energy levels in an atom

or molecule.

E

Photoelectron Auger electron

γ

Figure 5.4: Schematic picture of AES. In the first step, the sample is photoionized,and in the second step the Auger process takes place.

5.4 Near-edge Absorption Fine Structure (NEXAFS)

The fourth type of spectroscopy used in this thesis is Near-edge Absorption

Fine Structure spectroscopy, or NEXAFS. This kind of spectroscopy measures

the x-ray absorption near an ionization threshold. In our case, this method was

used to map out unoccupied electron orbitals, that is, eigenstates of the sys-

tem wavefunction that are not populated in the electronic ground state. Com-

monly, NEXAFS is used in surface science to determine molecular orientation

and symmetry in adsorbed molecules, since the technique is sensitive to bond

angles. With the equipment at our disposal, we measure what is known as par-

tial electron yield (PEY). This works in a way that the exciting photon energy

is swept over a certain energy region, and for each step in the sweep, most

46

of the electrons that are emitted, regardless of their source channel, will be

collected and summed by the electron spectrometer. When this sum is plotted

against the photon energy, one obtains the a spectrum that is approximately

proportional to the “real” absorption intensity.

5.5 Resonant Auger Spectroscopy (RAS)

Resonant Auger spectroscopy works much like AES, with the notable excep-

tion that there is no core ionization preceding the Auger decay, but rather a

core excitation. In such an event, a core electron is excited by a photon with

just the right energy to move it out of the core, and into one of the unoccu-

pied orbitals that was mentioned earlier in the description of NEXAFS. When

in such an orbital, the atom or molecule will be in a similarly unstable state

as when core-ionized, but the ways to a more stable situation are somewhat

different. There are two main cases, both of which have the common denomi-

nator that a valence electron falls down to fill the core hole. The first of these

is where the excited electron takes part in the core hole decay, leaving the

atom or molecule in a one-hole state. This is called a participator decay. The

second case is one where a valence electron is emitted as the Auger electron,

leaving the atom or molecule in a one-particle, two-hole state. This is called

a spectator decay. A schematic picture of RAS is shown in figure 5.5. In this

thesis, RAS has been used as a probe of molecular dissociation, and it will be

further described in this context in the results section.

E

Spectator decay Participator decay

γ

Decay types

Figure 5.5: Schematic picture of RAS. The shaded box shows the two different decaychannels, participator and spectator decay, that succeeds the core-excitation.

47

5.6 Interpretation of X-ray spectra

The interpretation of photoelectron spectra is not always a straightforward

task. There may be overlapping or unresolved features, making an unambigu-

ous interpretation of the spectra hard, but with modern tools many of these

difficulties may be resolved. Two common approaches when it comes to inter-

pretation is to either use curve fitting, based upon previous empirical knowl-

edge and known formulas for a given system, or to use ab initio calculationsto predict what should be seen in the spectra (binding energies, lineshapes and

linewidths, differential and total cross-section) and then assign features from

this calculation. There are several ways and methods to perform such calcu-

lations, and if such a method was used it will be described in greater detail in

context of the experiment it was applied to. If nothing else is mentioned, the

spectral analysis was done by curve-fitting. For the sake of simplicity, only

UPS and XPS will be shown in the following discussion. Interpretation of

RAS, AES and NEXAFS spectra will be discussed together with the results.

5.6.1 Koopmans theorem and the chemical shift

A theorem that is commonly used when interpreting photoelectron spectra is

Koopmans theorem [40]. According to this theorem, the ionization energy of

a molecule is equal to the energy of the highest occupied molecular orbital

(HOMO);

EB,k �−εk (5.1)

This allows for ab initio calculation of ionization energies of molecules,since orbital energies can be calculated using methods such for example den-

sity functional theory (DFT) or Hartree-Fock (HF) theory. This theorem does

not take electron correlation or relaxation of the orbitals upon ionization into

account when determining ionization energies, which is a problem in many

systems. However, the relaxation of orbitals upon ionization gives a contribu-

tion to the binding energy that, in many cases, is of opposite sign from that

of electron correlation. Relaxation effects come in many forms; but primar-

ily from two sources. These are relaxation of orbitals in the same atom or

molecule, and charge transfer to or from the surrounding atoms or molecules.

Because of the cancelling character of the relaxation and electron correlation

effects, Koopmans theorem often gives sufficiently good results for spectral

interpretation where there are clearly observable features in the spectra.

One of the most prominent features of photoelectron spectroscopy is its

inherent element and geometric sensitivity. This sensitivity is due to the

so-called chemical shift, which was mentioned earlier. The chemical shift ismainly due to what is known as screening, i.e. differences in the Coloumb

interaction of the vacancy in the ion with the rest of the atom or molecule. A

conceptually simple example is the sodium azide molecule, NaN3. The ionic

character of the various parts of this molecule (two N−, one N+ and one Na+

48

part) give rise to three distinct spectral features. One from the N+ ion, one

from both of the N− ions and one from the Na+ ion. The negative charges

screen the charge from the nitrogen nucleus, reducing the binding energy on

the negatively charged ions; this makes it possible to assign features even

from atoms of the same element, which differ only in position in a molecule.

In a covalently bound system, the chemical shift is mostly related to the

electronegativity difference of various parts of a molecule.

5.6.2 Atomic photoelectron spectra

The simplest case to begin with when discussing the interpretation of pho-

toelectron spectra is the atomic photoelectron spectra. As mentioned earlier,

molecular spectra can be said to be derived from the atomic spectra, and clus-

ter spectra in turn can be understood from the spectra of its constituents. An

ordinary atomic UPS spectrum is shown in figure 5.6a. Here, one clearly sees

two sharp peaks. These can be assigned to an atomic orbital which is spin-

orbit split, and the difference in energy between the two peaks is the differ-

ence in energy that the photoemitted electron has depending on its spin state.

In atomic XPS spectra, the situation is much the same. There is, however

one more thing that one needs to consider in XPS, namely the phenomenon

known as Post-Collision Interaction, or PCI [41, 42]. This is a mechanism that

introduces a kinetic energy dependent asymmetry in many XPS spectra. In a

classical picture, this can be viewed as a consequence of the outgoing photo-

electron being overtaken by the Auger electron after some time. The asymme-

try becomes larger if the photoelectron has a low kinetic energy, meaning that

the photoelectron is overtaken at an earlier stage. Usually, one tries to avoid

this by choosing the photoelectron kinetic energy to be large enough to avoid

the photoelectron being overtaken at all.

5.6.3 Molecular photoelectron spectra

Molecules differ from atoms in that they have at least two nuclei. These nuclei

move with respect to each other depending on how the electron cloud is cur-

rently configured, by vibrations and rotations along and around the bonds in

the molecule. This gives rise to vibrational features in the molecular spectra,

as is shown in 5.6b, as was discussed earlier. From such spectra it is possible

to deduce for example the bond-lengths in a molecule. The relative probability

of a vibrational transition is given by the Franck-Condon factor (see eq. 3.2).

From these it is possible to deduce various things about the molecule, such as

the shape of its potential surfaces. In XPS, the main difference between the

atom and the molecule is that the molecule may have many different core-

levels, each situated in an individual atom in the molecule. These may differ

much in energy, such as the levels C1s and O1s levels in CO, or have lev-els that are close to each other like the two N1s levels in N2O. One of the

49

1516

a b

13 12

hν=61 eVhν=61 eV

Binding energy (eV) Binding energy (eV)

Rel

ativ

ein

tens

ity

(arb

.un

its)

Figure 5.6: UPS spectra of the atomic Ar 3p state (panel a) and of the O2 X-state(panel b). The two peaks in the Ar spectrum are due to the spin-orbit splitting of the

atomic 3p level. The corresponding term symbols are 2P1/2 and 2P3/2 from left to

right. The peaks in the O2 spectrum are due to the intramolecular vibrations.

strongest points with XPS is that it can separate the core levels for each in-dividual inequivalent atom in the sample, by the chemical shift induced by

the neighboring atoms. XPS is, for example, sensitive enough to differentiate

between the core level of the far-end N and the central N in N2O. This fact

can be, and is commonly, used to fingerprint molecules, as mentioned earlier

in section 5.2.

5.6.4 Cluster photoelectron spectra

In addition to all the mechanisms mentioned above, clusters have some more

things that complicates the spectral interpretation. In the systems presented in

this thesis, the main difference between the previous cases is that the neigh-

bors in a cluster induce additional polarization screening of the core hole whenphotoionization occurs [43]. Disregarding initial and minor final state effects,

this means that the effective binding energy of the electrons is lower in a clus-

ter of a given atom or molecule than in the free ditto. The screening is, to

a large extent, determined by the nearest neighbor atoms or molecules. This

leads to that vacancies in bulk atoms are screened to a higher degree than

in surface atoms, since bulk atoms have more nearest neighbors. Figure 5.7

shows a schematic picture of the polarization screening mechanism. In core-

level photoelectron spectra of rare gas clusters, this is seen as two distinct

features of different binding energy, the surface feature being closest to the

atomic feature. This is mainly used to study the difference between surface

and bulk properties, but can also be used to determine approximate cluster

size [44].

50

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ --------------

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

hν e−

Figure 5.7: A schematic picture of the screening of an electron (core) hole in an in-sulating van der Waals cluster. The positive charge is screened by electrons of neigh-

boring particles being polarized towards it.

Clusters are also, though very small, extended systems. That means that

there is an effective electron attenuation length, which is the distance an elec-

tron statistically travels inside the cluster before being absorbed or scattered

by another atom. This attenuation length is strongly dependent on the electron

kinetic energy [45, 46]. This dependence is often used to vary the contrast

between the cluster surface and the cluster bulk contributions to the spectrum.

Last, cluster features are broader than atomic or molecular features. This

is due to various mechanisms [47], but mainly to variations of polarization

screening depending on the position of the ionized atom. This is due to the

fact that the sample beam that we probe consists of clusters of a lot of dif-

ferent sizes, as has been discussed earlier. Furthermore, the clusters do not

always form as perfect crystals, but rather as a mix between locally well or-

dered chunks and other more randomly ordered parts. The final spectral shape

of clusters will be an average with contributions from all of the possible clus-

ter sizes and geometries. There are other, minor effects which also broaden

the cluster spectra; for example vibrations in the clusters. All this considered,

one has a fairly good idea of how to decompose a cluster spectrum. There are

also recent advances in the theoretical lineshape modeling of clusters that can

potentially ease the burden of cluster scientists by predicting the cluster size

theoretically from experimental spectra [48, 47].

51

6. Summary of papers

In this section, a summary of the results of all the papers will be presented,

together with a more detailed discussion of the different experiments and the

theory employed to interpret the results.

6.1 Dissociation of molecules

In papers I-III, the dissociation behavior of different molecules (O2, CH3Brand Na2) in different media (O2 clusters and Ar clusters) and in vacuo re-spectively is investigated. In section 3.2, potential curves of molecules were

discussed. These all showed bound states, i.e. states in which, without vibra-

tional excitation, the intra-molecular bonds are not broken. Keeping this dis-

cussion in mind, there are also electronic states in molecules which are purely

repulsive. Figure 6.1(a) shows a schematic comparison between a bound state

potential and a repulsive potential. The steepness of the dissociative potential

in the FC-region will, in principle, determine the time duration of the disso-

ciation. By experimentally measuring the kinetic energy release (KER) of the

dissociation, such potential curves can be characterized [49, 50].

Internuclear distance (A)

Pot

ential

ener

gy(a

rb.

unit

s)

1s−1

σ∗

X 3Σ+

g

0 1 2 3 4 5

FC region

(a) A schematic picture of the ground

state potential and the 1s−1σ∗ excitedstate potential

Inte

nsity

/arb

.un

its

Photon Energy/eV

3σ∗

Molecular TIY

Cluster PEY

530 535 540 545

3π∗

1 2 3

<N>=10000

(b) The x-ray absorption spectrum of

free oxygen molecules (bottom) and clus-

tered oxygen molecules (top).

Figure 6.1: A dissociative potential curve and the oxygen cluster NEXAFS spectrum

A way of preparing such a dissociative state is to perform a core-to-valence

excitation. This means that one excites an electron from a core-orbital to an

unoccupied valence level, by absorption of an x-ray photon as in for exam-

53

ple RAS spectroscopy (see section 5.5). A molecule in such an excited state

will, in some cases, dissociate on a fs time-scale. This means that the disso-

ciation takes place on the same time-scale as an Auger decay. Breaking of

the molecular bond on this time-scale is often termed ultra-fast dissociation

(UFD) [51, 52, 53, 54]. The consequence of this is that one can observe spec-

tral features from Auger decays both in the intact molecule and from decays

taking place in fragments of the molecule. This type of spectroscopy has been

performed in papers I and II. Both of these papers address the question ofwhat effect the surrounding medium has on the dissociative character of the

core-excited state.

6.1.1 Oxygen clusters

In paper I, clusters of oxygen have been studied in an attempt to quantify theeffect of clustering on dissociation after core excitation. Oxygen was chosen

as an experimental system, since it is fairly easy to handle and since here are an

abundance of data already published for the molecule in vacuum and in solid

phase [54, 55]. NEXAFS spectroscopy was employed to locate the 1s−1σ∗

resonance in the clustered molecules, the spectrum is shown in figure 6.1(b).

ΔEv = 0.8 eVΔEv

Inte

nsity

/arb

.un

its

Binding Energy/eV

13 11

546 544 542

ΔEc

ΔEc

ΔEc = 0.7 eV

(a) UPS and XPS spectra of oxygen clus-

ters. The polarization screening shift was

found to be 0.7-0.8 eV. In the UPS spec-

trum (top) the molecular progression is

the spectral feature with the highest bind-

ing energy, and the broad feature at lower

binding energy is the cluster feature. The

XPS spectrum is fit by a molecular com-

ponent and a cluster component.

470 475 480 485 490 495Kinetic Energy/eV

Inte

nsity

/arb

.un

its

ΔE

ΔE

ΔE= 1.0 eV

2P

2D

1

1

2

3

(b) RAS spectra of oxygen clusters. Ex-

citation energies are marked by the num-

bers 1 through 3, and match the ver-

tical bars in figure 6.1(b). The bottom

spectrum is the RAS spectrum of the

free molecule. The other spectra are from

the clustered molecules; for each energy

point there two spectra are displayed. The

topmost of these two is the total spec-

trum while the lower one shows the clus-

ter spectrum only (where molecule has

been subtracted).

Figure 6.2: UPS, XPS and RAS spectra of oxygen clusters.

54

In the NEXAFS spectrum it can be seen that the shape and position of the

1s−1π∗ resonance is virtually unaffected by clustering. This is not the case forthe 1s−1σ∗ resonance, which is somewhat blue-shifted and broadened in theclusters. Figure 6.2(a) shows UPS and XPS spectra of oxygen clusters, which

were used to extract the shift due polarization screening for a singly charged

molecule in the cluster matrix. The shift turned out to be around 0.7-0.8 eV in

these spectra.

A detuning study was performed to characterize to 1s−1σ∗ resonance in theclusters. In the resulting RAS spectra, which are shown in figure 6.2(b), one

can see that there are spectral features that are shifted towards a higher kinetic

energy (i.e. a lower binding energy) relative to those corresponding to decay in

an atomic oxygen fragment in the case of the free molecule. The shift towards

higher kinetic energy is approximately 1 eV. This is slightly larger than the

polarization screening shift for a singly charged molecule seen in XPS and

UPS. An explanation for this difference is that the excited molecule breaks

into fragments which have time to move inside the cluster matrix; a typical

path-length of 1 Å before the Auger decay takes place can be calculated for

the excited fragment. Since this, on average, moves the fragment closer to

its neighbors, the screening increases. These facts, when taken together, are

very strong indications of the UFD process taking place inside the clusters. In

this case it was not possible to distinguish contributions from the surface/bulk

components, due to the very demanding experimental conditions. What one

can observe, though, is that there seems to be no radical differences between

the molecular and the cluster RAS spectra, save that of the shifted features that

can be attributed to polarization screened fragments. These findings indicate

that the valence electronic structure of oxygen molecules is not affected by

clustering to any significant amount.

6.1.2 Bromomethane clusters

Paper II presents a similar study as that of paper I, with the principle dif-ference that bromomethane (CH3Br) clusters have been used in the sample

beam. The extent of the CH3Br valence orbitals is larger than that of O2, and

the interaction between the molecules is governed not only by van der Waals

forces, but also by dipole-dipole interactions. This gives the CH3Br clusters

a local ordering of Pnma packing [56], in contrast to the rather amorphous

packing of oxygen molecules under the experimental conditions employed in

paper I [57]. To characterize the band formation in bromomethane clusters,we have performed ab initio model calculations on the bromomethane dimer.We have utilized the geometries found in [58], and performed anMP2 calcula-

tion, using the LANL2 effective core potentials [59] together with a cc-PVDZ

basis set. All of the calculations were performed usingGaussian 03c [60]. Thesplitting of the outermost molecular 2e valence level due to dimerization canbe viewed as a lower limit to the band-width in the larger cluster system. It

55

is found that the splitting in the dimer becomes � 0.2 eV. The calculations

show also that the valence state in the dimer is formed by orbitals that are

delocalized over neighboring molecules. This makes it rather probable that

even more overlap will occur in a cluster matrix, where there are more nearest

neighbor to each molecule. A picture of a resulting electronic orbital in the

bromomethane dimer is shown in figure 6.3.

Figure 6.3: A resulting molecular orbital (HOMO-3) in the dimer.

The onset of band formation means that the valence electronic structure

is very much affected by clustering. Thus, there might be significant differ-

ences between the cases of bromomethane and oxygen. In the case of bro-

momethane, the core-to-valence excitations of interest are 3d5/2 → 4a1 or3d3/2→ 4a1. Figure 6.4(a) shows the NEXAFS spectra of molecular and clus-tered bromomethane. In spite of the apparent similarity of the 3d → 4a1 ab-sorption spectrum in the molecular and cluster cases, we observe significant

differences in the character between the two, when analyzing the RAS spectra

at different points on the resonance.

In the molecule, both of the spin-orbit states are dissociative. There have

been previous studies on the bromomethane molecule to map this shape reso-

nance [52, 53]. Figure 6.4(b) shows molecular RAS spectra, at different exci-

tation energies. The spectral shape has been assigned in the previous studies

as being due to a mixture of molecular Auger features and atomic Auger fea-

tures. The shift between the two spectral features at the higher and the lower

excitation energies can be attributed to the difference in energy between the

two excited states. The spin-orbit splitting between the 3d5/2 and 3d3/2 levelshas been found to be approximately 1 eV, which is consistent with what has

56

69.5 70.0 70.5 71.0 71.5 72.0 72.5Photon energy (eV)

Inte

nsity

(arb

.un

its)

3d5/2 → σ∗ 3d3/2 → σ

∗

ΔE

(a) NEXAFS of bromomethane

molecules and clusters. Within the

measured statistics, no change in the

peak shape can be observed.

50 52 54

1D

1D

3P

3P

3d3/2 → σ∗

3d5/2 → σ∗

hν=70.2 eV

hν=70.6 eV

hν=71.0 eV

hν=71.4 eV

hν=71.8 eV

Kinetic energy (eV)

Inte

nsity

(arb

.un

its)

(b) CH3Br molecular RAS spectra.

Atomic features are shown as solid lines,

as is the total fit lineshape. The shaded ar-

eas represent molecular Auger features.

Figure 6.4: The NEXAFS spectrum of molecules and cluster, and the RAS spectra ofthe molecule, detuned over a range of energies as indicated in the figure.

been observed in earlier works [52, 61]. This splitting corresponds very well

to the Auger feature shift when exciting the two spin-orbits components.

Figure 6.5(a) shows the XPS spectrum of bromomethane clusters of two

different sizes. These have been used to extract the shift due to polarization

screening for a singly ionized state.

In clusters, if the electronic final state is localized to an individual molecule,

the vacancy will be affected by polarization screening. If this is the case, and if

UFD takes place in the bromomethane clusters, one should be able to observe

spectral features from atomic fragments that are shifted up in kinetic energy.

Figure 6.5(b) shows cluster RAS spectra at the same excitation energies as for

the molecule. These have been treated within the framework proposed above,

i.e. by modeling the cluster lineshape as a shifted and broadened copy of the

features stemming from the free molecule. As can be seen from the figure,

this kind of model works rather well to describe the experimental spectrum

for both of the spin-orbit components.

However, upon analysis of the ratios between decays in the molecule and

the cluster, and between the resonant Auger decays and photoionization, one

comes to several conclusions. There is a clear effect on the dissociative char-

acter of the 3d → 4a1 excitation in clusters when compared to the same exci-tation in the free molecule. The number or decays in screened fragments, i.e.

atomic fragments that are within the cluster matrix is suppressed compared to

the number of decays in fragments from free molecules. Also, the total num-

ber of decays observed in the RAS spectra is suppressed as compared to the

57

hν=110 eV

hν=110 eV

hν=110 eV

ΔEL=0.81 eV

ΔEL=0.72 eV

Inte

nsity

(arb

.un

its)

Binding energy (eV)78 77 76 75

Large clusters

Small clusters

Molecule

(a) XPS spectra of the CH3Br molecule,

small clusters and large clusters. The po-

larization screening shift was found to be

0.72 eV for the small clusters and 0.81

eV for the larger size.

50 52 54

1D

1D

3P

3P 3d3/2 → 4a1

3d5/2 → 4a1

hν=70.2 eV

hν=70.6 eV

hν=71.0 eV

hν=71.4 eV

hν=71.8 eV

Kinetic energy (eV)

Inte

nsity

(arb

.un

its)

(b) The RAS spectra of large clusters,

at the same excitation energies as those

used in 6.4(b).

Figure 6.5: The XPS spectra from the molecule and two sizes of clusters and the RASspectra from large size clusters.

RAS spectra of the free molecule. The latter fact is somewhat surprising, since

the NEXAFS spectra are very similar.

Several possible explanations exist for these observations. However one can

rule out some explanations, like electron recapture, fragment cage exit, neigh-

bor closing and secondary ionization due to various reasons. Remaining expla-

nations include band formation in the initial and final states (charge transfer)

or physical differences between the two spin-orbit components induced by ge-

ometry, for example variations in the molecular field splitting components. As

can be seen, all of the valence states in the dimer are delocalized to some ex-

tent, so it is not unreasonable to believe that this might be the case also for the

RAS intermediate states. Such delocalization would explain the effects seen

above, since it would imply that many of the excited electrons are delocal-

ized, thereby forming an, in principle, core ionized final state which would

then undergo normal Auger decay. Normal Auger features are not visible in

our RAS spectra because of the narrow energy region we examine. However,

they will be counted when performing NEXAFS spectroscopy, which makes

this a plausible explanation for what is seen in the spectra.

58

6.1.3 Sodium dimers

Another way of inducing dissociation of a molecule is by performing core

photoionization. A core-ionized, bound state can relax by an Auger process.

The final state can then be dissociative. In a spectrum where Auger electrons

are measured, a dissociative state will give rise to a broad, featureless bump.

The width of this bump can be related to the slope of the potential surface

of the final state. Figure 6.6(a) shows a schematic picture of this relationship.

Such an analysis was performed in paper III. A beam of sodium dimers (or,really, Na2 molecules; the dimers are covalently bound with the outermost

3s electrons forming a binding molecular σ orbital) was chosen as a proto-

type system, since its electron structure is very simple, with only two valence

electrons, and since it is a case where, in the atom, Auger decay from the

core-ionized 2p−1 state is not allowed while it becomes possible in the dimer.

1 2 3 4 5

FC-region(ground state)

FC-region(excited state)

Augerwidth

Final state

Ground state

Excited state

Ene

rgy

(arb

.un

its)

Interatomic distance (A)

EffectiveFC-region

0

1

(a) A schematic picture of how the

width of an Auger feature can be esti-

mated from knowledge about the poten-

tial curves of a molecule. In this case, an

“effective” FC-region was defined, to ac-

commodate the fact that the transition to

the dissociative state takes place from an

excited state.

Dimer feature

16 18 20 22 24

Inte

nsity

(arb

.un

its)

Kinetic energy (eV)

Auger width

1,3S 3P

0

1

(b) The Auger spectrum from the 2p-ionized Na2 molecule. The width of the

Auger feature is well estimated using the

model with an effective FC-region.

Figure 6.6: Potential curves and Auger spectrum for the 2p-ionized Na2 molecule.

In this work, we have modeled the Auger final state by a pure Coulomb

potential on the form

Vcol =q1 ·q2

r(6.1)

where the charges q1 and q2 were each set to one. This is based on the as-sumption that, in the final state, the two positive charges in in the valence

orbitals repel each other and end up on different Na atoms in the molecule.

Using the effective-FC-region approximation, we come to the conclusion that

59

a pure Coulomb potential represents the Auger final state very well in this

case. Another point in favor of this conclusion is the fact that the Na+ ions in

the final state have smaller radii than the the neutral atoms that constitute the

molecule; thus the overlap between the outermost Na orbitals will be much

smaller in the final state than in the initial state (2.32 Å [12] as compared to

3.08 Å [62]).

The main point of this paper, however, is to assign the molecular field split-

ting and life-time of the Na2 molecule Na 2p core hole. This was done usingXPS, in combination with ab initio calculations. Density functional theory(DFT) [63] was employed to calculate the inter-nuclear distance in ground

state, and in the core-ionized state (using the Z+1 approximation [64]). The

experimental spectrum was subsequently fitted using the calculated vibra-

tional progressions. Figure 6.7(a) shows 2p binding energy region from boththe atom and the molecule, while figure 6.7(b) shows only the molecular spec-

trum with the fit components. The molecular field splitting was determined to

be 42±10 meV and the core-hole lifetime was found to be 15±8 fs from theLorentzian width of the fitted peaks. The spin-orbit splitting corresponds well

to previous studies of solid sodium [65], with a value of roughly 160 meV.

However, the lifetime of the core-hole is much shorter than what has previ-

ously been reported [66]. Even though many competing processes take place

in the solid state that can affect the lifetime, this might indicate that the life-

time in the solid state might have been overestimated.

39 38.5 38 37.5 37 36.5 36

x 15

1P1

3P0

3P1

3P2

2p1/2

2p3/2

hν = 61 eV

Inte

nsity

(arb

.un

its)

Binding Energy (eV)

1

0

(a) An overview spectrum of the 2p bind-ing energy region for the Na atom and for

the Na2 molecule.

36.3 36.2 36.1 36 35.9 35.836.4

hν = 61 eV

2p1/2

2p3/2

Inte

nsity

(arb

.un

its)

Binding Energy (eV)

Em

0

1

(b) The XPS spectrum of the molecule.

The solid line represents the fitted line-

shape, the dotted lines represent the in-

dividual fit components and the vertical

bars represent the vibrational bar spec-

trum.

Figure 6.7: XPS spectra from the sodium atom and the sodium dimer molecule.

6.1.4 Conclusions

Synchrotron radiation based techniques were used to probe dissociation

dynamics in various systems. It was shown that dissociative states of

60

molecules in the free phase is not necessarily dissociative when the same

kind of molecule is in a cluster. The reason for this is, as of yet, not entirely

clear, but with more theoretical effort and better experimental set-ups, the

future of this kind of experiments is very promising. It was also shown that it

is possible to determine the core-hole lifetime and molecular field splitting

in alkali-metal dimers with a good accuracy. This is highly interesting, since

there are no solid-state limitations and effects to take into account when

performing the analysis of effects such as molecular field splitting and

lifetime.

6.2 The structure of doped and molecular clusters

Paper IV-VI deals with the geometric structure of doped and pure free molec-ular clusters. Not much is known about these structures a priori, since it isrelatively recently that experimental and computational tools have become

available that allow for reasonably accurate structure determination of clus-

ters [47, 57, 67, 68, 69]. Performing calculations on large clusters (several

thousands of atoms or molecules) is still prohibitively computationally ex-

pensive, but the feasible size limit has been pushed upwards to at least a few

thousands of atoms or molecules. In such calculations, a classical or semi-

classical approach is most often taken, using molecular dynamics (MD) or

Monte-Carlo methods to predict the geometric structures of clusters. Limita-

tions to these models are mainly that electronic effects, such as magnetism, are

not properly taken into account. This is not always a problem, but, for exam-

ple, in the case of oxygen clusters, the paramagnetic nature of the constituent

molecules dramatically influences the phase diagram [57].

In paper IV, the diffusion behavior of oxygen molecules adsorbed on large(N � 〈8000〉) Ar host clusters has been investigated. There are many compre-hensive theoretical studies of the diffusion behavior of atoms and molecules

doped on inert host clusters [68, 69], and also a range of measurements per-

formed by fluorescence spectroscopy [34, 35]. Recently, XPS spectra of polar

molecules adsorbed on Kr clusters were also published [70]. Figure 6.8(a)

shows the X-state UPS spectrum of oxygen molecules which are doped on

Ar. An important point here is that intra-molecular vibrations can still be re-

solved even in the cluster feature. This indicates that there is no significant

band formation between the different oxygen molecules in this case. That it is

the case does not come as a surprise, since the oxygen valence levels are fairly

confined to the molecule, and since the oxygen molecules probably are fairly

sparse in the Ar matrix.

This allows us to use UPS as our structural probe, instead of using XPS,

which is the most commonly used technique for this purpose [47, 48, 71, 72,

73, 74, 75, 75, 76]. Using UPS has several advantages, especially in the case

of oxygen. First, the photon flux at the beamline where the experiment was

61

13 12.5 12 11.5 11 10.5

Binding energy (eV)

Inte

nsity

(arb

.un

its)

hν=60 eV

A

B

Pd=18 mbar

*

(a) The O2 X-state in O2/Ar clusters. The

vertical solid bars mark the molecular FC

profile. The dashed vertical bars mark the

same FC profile shifted to a lower bind-

ing energy to try to match the cluster

feature. The * marks the region of the

spectrum which cannot be described us-

ing just one vibrational progression.

Vertex

Edge

Face

Interface

Bulk

13 12.5 12 11.5 11Binding energy (eV)

Inte

nsity

(arb

.un

its)

Pd=9 mbar

Pd=14 mbar

Pd=18 mbar

Pd=29 mbar

hν=60 eV

(b) Spectra of the O2 X-state in the

O2/Ar clusters at different doping con-

ditions. The shaded areas represent ion-

ization at different sites in the cluster. As

the doping pressure increases, the black

(bulk) feature grows dramatically.

Figure 6.8: UPS spectra of the O2 X-state in O2/Ar clusters.

performed is optimal at energies that access the valence region. Second, the

resolution is not limited by the lifetime of the vacancy, since the typical decay

pathway of a valence ionized state is radiative decay. In the case of oxygen,

it is very hard to probe the core-levels with an acceptable resolution and still

have a sufficient photon flux at the beamline utilized in this experiment. A

complicating fact is that the valence states are quite rich in vibrational struc-

ture; however, this is not such a big problem as one might first think, since the

valence spectra of molecules are often well known and extensively studied.

From figure 6.8(a) it can be seen that the simple shift-and-broadening ap-

proach that is usually applied in the case of XPS does not work very well

to describe the oxygen cluster spectral feature. Instead, we have employed

a modified version of this approach. To be able to quantify where the oxy-

gen molecules are sited in the Ar matrix, we have used electrostatic calcu-

lations (Tinker package, using the AMOEBA polarizable forcefield [77]) to

get the polarization screening shift values for different sites in and on the

Ar cluster. With the help of depth-profiling, (pure) surface/bulk splitting has

been ruled out as the source of the discrepancy between the simplest shift-

and-broadening model and the experimental spectrum at low pressures. Fig-

ure 6.8(b) shows oxygen cluster UPS spectra as a function of the doping pres-

sure. As the doping pressure increases, it can be seen from the fits that the

62

feature at the bulk energy (black) increases drastically. This is expected, since

at some point, the Ar host cluster will become liquid, and oxygen molecules

will, in principle, be able to migrate freely in the Ar host. At lower doping

pressures, the oxygen molecules does not penetrate into the bulk at all, but

stay in the surface and interface layers.

In general, the results of this paper agrees well with how one would expect

that oxygen behaves when doped on Ar by considering its van der Waals in-

teraction potential. That no band formation is observed is a fact that can allow

UPS to be used as a structural probe in many other mixed cluster experiments

with similar properties. The results of this study indicate that a high degree of

control of the radial structure and diffusion behavior of molecules adsorbed

on inert host clusters can be achieved.

Paper V presents a doping study, using XPS of Ar clusters doped with Kr

and Xe. This technique has the great advantage that it provides a (in principle)

linear relation between the observed spectral intensity and the actual com-

position of the sample. This is a modified truth, because of issues regarding

the electron effective attenuation length of electrons in the cluster [45, 46].

Figure 6.9 shows XPS spectra of the two systems, at different doping condi-

tions. One can clearly see the evolution of the dopant features in these spec-

tra. For Kr, there is a clear radial structuring, where most of the Kr ends up

on the surface of the cluster, in contrast to the co-expanded case, where Kr

atoms are mostly found in the bulk, and Ar atoms are mostly found on the

surface [73, 74].

surface

surface

bulk

bulk

0.0 0.0-1.0 -1.0

pure

pure

doped

doped

co-e

xp

co-e

xp

Ar 2p−1

3/2 XPS, hω = 270 eV Kr 3d−1

5/2 XPS, hω = 115 eV

Inte

nsi

ty[a

rb.u

nit

s]

Inte

nsi

ty[a

rb.u

nit

s]

Relative binding energy [eV ] Relative binding energy [eV ]

(a) XPS spectra of Ar clusters doped and

co-expanded with Kr.

-1.00.00.0 -0.5-0.5 -1.0

pure

pure

Doped:

40

mbar

Doped:

40

mbar

Doped:

2m

bar

Doped:

2m

bar

co-e

xp

co-e

xp

Ar 2p−1

3/2 XPS, hω = 270 eV Xe 4d−1

5/2 XPS, hω = 112 eV

Inte

nsi

ty[a

rb.u

nit

s]

Relative binding energy [eV ]Relative binding energy [eV ]

(b) XPS spectra of Ar clusters doped and

co-expanded with Xe.

Figure 6.9: XPS spectra of Ar/Kr and Ar/Xe clusters.

For Xe, the situation is more complex. At low doping pressures, the dopant

remains in the surface layer, with a polarization screening that can be traced

to the Ar matrix. For higher doping pressures, the Xe XPS feature looks very

much like the pure Xe clusters, which are also shown in the figure. This has

been interpreted as island formation on the Ar surface; a schematic picture of

this is shown in figure 6.10. This is also very different from what is seen from

63

the co-expanded case, where there is a clear interface feature in the spectra,

indicating a very high radial segregation [72].

Low doping pressures High doping pressures

Figure 6.10: A schematic picture of how Xe forms islands as it aggregates on the Arclusters.

The main conclusion is that it is, indeed, possible to create free clusters

by post-expansion doping that do not have a thermodynamically favorable

structure, such as has been predicted in, for example, [68]. The other method

of creating mixed rare-gas clusters, co-expansion, gives a cluster distribu-

tion in which clusters have a much more thermodynamically favorable struc-

ture [72, 73, 74]. This is not surprising, considering that the cluster flight dis-

tance from the expansion nozzle to the probing region is in the order of several

cm, allowing the clusters time to cool down and minimize their energy by at-

taining a more stable configuration. In the doped case, the situation is such

that the host clusters have time to cool down even before the doping takes

place. Thus the doping process might or might not heat the host cluster up to

the melting temperature, allowing for control of the radial distribution of the

dopant atoms in the Ar host.

Another subject about which very little is known is the structure of molec-

ular clusters. These clusters can be formed by a combination of a wide range

of inter-molecular forces, such as van der Waals forces, dipole interactions

or hydrogen bonds. In paper VI, the structure of CH3Br clusters was studiedby XPS and UPS spectroscopy. The polarization screening shift value trend

in both XPS and UPS follow a similar pattern; the orbitals that are localized

on the methyl group have a larger polarization screening shift, while the ones

that are localized on the bromine atom have a smaller polarization screening

shift. States involving orbitals that are delocalized over the molecule have a

polarization screening shift in between the two. Figure 6.11 shows UPS and

XPS spectra from large bromomethane clusters. The shift values are marked

in the figure.

These shift values tell us that the bromine end of molecules are mostly

screened by methyl groups, which have a small polarizability compared to

bromine atoms. It also tells us that the methyl carbon is mostly screened by

bromine atoms, which are highly polarizable. All things considered, this is

strong evidence for a packing in the cluster matrix similar to that of the bulk

material [78], and to that which has been proposed for the bromomethane

dimer [58]. Recent calculations on the local structure of bromomethane clus-

ters also support these findings [79].

64

Inte

nsity (

a. u

.)

78 77 76 75Binding Energy (eV)

3d3/2

≈ 0.94 eV

(a) XPS Br 3d

74

3d5/2

≈ 0.94 eV

Molecule

Cluster

293 292 291 290Binding Energy (eV)

(b) XPS C 1s

Inte

nsity (

a. u

.)

≈ 1.25eV

Molecule

Cluster

(a) XPS spectra of the Br 3d and C 1sedges in bromomethane clusters.

2e

11.0 10.0 9.0

Binding Energy (eV)

10.5 9.5 8.5

Inte

nsity (

a. u.)

≈ 0.9 eV

11.0 10.010.5

(a) UPS

Molecule

Cluster

18 16 14Binding Energy (eV)

17 15 12

Inte

nsity (

a. u.)

17 1416

(b) UPS

13

15 13

1e

3a1

Molecule

Cluster

≈1.1 eV ≈1.0 eV

(b) UPS spectra of the 1e, 3a1 and 2e va-lence states of bromomethane clusters.

Figure 6.11: XPS and UPS spectra from pure bromomethane clusters.

6.2.1 Conclusions

The structure of neutral free clusters is hard to determine using experiments,

but with the help of ab initio and MD calculations, XPS and UPS spectra cangive accurate information. The fact that the techniques are inherently surface-

sensitive give a distinct advantage when investigating the structure of nano-

scale objects such as clusters. In this work, the behavior of rare-gas atoms

and molecules adsorbed on rare-gas cluster hosts has been studied using these

techniques together, giving a more clear picture of what influences the final

structure of the cluster system. XPS has also been used to characterize the

structure of a cluster consisting of polar molecules; the same reasoning can be

applied to clusters of other polar molecules to investigate local geometry and

packing.

65

6.3 Laser excited metal vapors

Papers VII and VIII deal with x-ray photoelectron spectroscopy on vapors oflaser-excited sodium and potassium, respectively. In photoemission, there is

a type of processes known as electron shake. These shake processes can beclassified as either shake-off, shake-up or shake-down. Figure 6.12 shows a

schematic picture of the final states in each of these processes. In a shake-off

type process, the outgoing photoelectron loses enough energy to the valence

electronic configuration to cause valence ionization. Thus the final state con-

sists of a photo-electron, a shake-off electron and a doubly charged ion. A

shake-up process is similar to the shake-off in that the outgoing photoelectron

loses energy to the valence electrons. It does not, however, cause ionization,

but rather a valence excitation; i.e. a valence electron is excited into an unoccu-

pied orbital. Both of these processes reduce the kinetic energy of the outgoing

photoelectron [80]. The third possibility, shake-down, is only possible when

photo-ionizing an excited state. In this case, the photoelectron does not lose

energy to the valence electrons, but rather gains energy while a de-excitationof the initial state takes place.

E

Ground state Shake-up Shake-off Excited state Shake-down

hνhν

Ground state shake Excited state shake

Figure 6.12: Three different shake processes. On the left hand side, shake-up and

shake-off from the ionized ground state, on the right shake-down from the ionized

excited state.

The first experimental observations of the shake-down process are pre-

sented in paper VII. The relative simplicity of the sodium atom (only s andp orbitals, and only one electron in the valence shell) and the fact that the3s− 3p gap is in the optical range (588.995 nm, 2.1 eV) makes it an idealcandidate for studies of the shake-down process itself. It turned out to be pos-

sible to directly compare the direct photoemission lines from the ionization

2p63s + hν → 2p53s+ e− to the shake-down photoemission lines, 2p63p +hν → 2p53s+ e−. The difference in kinetic, and thus binding energy of the

66

two kinds of photoelectrons was shown to be 2.1 eV, corresponding to the

excitation energy of the laser light.

39 38.5 38 37.5 37 36.5 36 35.5binding energy (eV)

inte

nsity

(ar

b. u

nits

)

÷100

1P

1

3P

0

3P

1

3P

2

1P

1

3P

0

3P

1

3P

2

2.1 eV2.1 eV

(a) XPS spectrum which shows both the

ground-state spectral features and those

from shake-down satellites.

36.5 36.4 36.3 36.2 36.1 36 35.9 35.8 35.7binding energy (eV)

Inte

nsity

(ar

b. u

nits

)

45 degree fit atomic shake-down cluster direct 2p

-1

3P

2

1P

1

a)

(b) A closeup of the shake-down spec-

tral features. The dotted line represents

the direct photoemission from the Na2molecule.

Figure 6.13: Overview and detailed decomposition of the shake-down features of

laser-excited Na atoms.

Figure 6.13(a) shows the measured 2p XPS spectrum. At higher bindingenergy, the ground state multiplet features can be seen, at higher energies are

the shake-features; these are, as expected, a shifted copy of the ground state

multiplet lines. As can be seen from figure 6.13(b), there is another feature

overlapping the shake-down lines, this is attributed to the ground state 2pphotoemission from the Na2 molecule, as was discussed in paper III. To con-serve parity between the initial and final states, it turns out to be necessary

that the outgoing 2p photoelectron wave is p-symmetric, and that the excited3p electron undergoes a dipole transition to the 3s orbital. It was also possibleto probe the alignment of the excited atoms; it is described well by the frame-

work of linear dichroism [81]. The most prominent part in the description of

the dichroism in this case is the term which is independent on the relative po-

larizations between the laser light and the synchrotron light. This shows that

the photoelectrons are primarily emitted in the direction of atomic alignment.

Figure 6.14(a) shows the shake feature measured at different polarization an-

gles, and figure 6.14(b) shows the fitted dichroism function.

Shake-down was studied also in paper VIII, but with the purpose of char-acterizing final states that are only weakly excited in direct photoemission.

In this case, 3p photoemission from potassium atoms was examined. For thelaser part of the experiment, the exciting radiation was tuned to 769.9 nm,

1.61 eV. This pumped the K 4s 2S1/2 → 4p 2P1/2 transition. In the direct 3pphotoemission from potassium, there is a significant configuration interac-

tion between the 3p54s and 3p53d states. There are also a number of extrafinal states from the 3p53d configuration, namely the 3p53d 3F states. The

ground state of potassium is S-symmetric, and thus one would not expect tosee F-symmetric final states in the direct p-photoelectron emission. Even so,

67

36.5 36.4 36.3 36.2 36.1 36 35.9 35.8 35.7binding energy (eV)

Inte

nsity

(ar

b. u

nits

)

0o shakedown

45o shakedown

90o shakedown

135o shakedown

cluster direct 2p-1

3P

2

1P

1

b)

(a) XPS spectra of the shake-down fea-

ture at various laser polarization angles.

0 30 60 90 120 150 180Angle (deg)

Inte

nsity

(ar

b. u

nits

) 3P

21P

1

(b) The measured intensity as a function

of polarization angle.

Figure 6.14: Angle dependence of the shake-down spectral features.

such states are visible. Figure 6.15(a) shows the ground state 3p photoelec-tron spectrum. The observed F-symmetric features stem from configuration

interaction, shake-up photoemission or higher-order photoemission.

25.5 25 24.5 24binding energy (eV)

inte

nsity

(ar

b. u

nits

)

K 3p PES× 200

3p54s

3P

2

3p53d

3p54s

1P

1

3p5(3d 4s)

1 3P

3F

2

3F

3

3F

4

(a) Ground state K 3p direct photoe-

mission spectrum. The dashed line is

the same spectrum as the solid line, but

scaled by a factor of 200 to show the very

weak F-symmetric final state features.

25.5 25 24.5

inte

nsity

(ar

b. u

nits

)

24 23.5 23binding energy (eV)

inte

nsity

(ar

b. u

nits

)

ground state K

laser excited K

3P

1P

3F

(b) Comparison between the shake-down

features (lower panel) and the direct 3pphotoemission. The relative intensity of

the F-symmetric final states to the P-

symmetric final states varies drastically

between the two cases.

Figure 6.15: XPS spectra of the K3p level.

We have also observed the 3p53d 1,3D lines in the direct photoemission

spectrum. In the shake-down spectra, a difference in excitation scheme from

the direct photoemission case (in direct photoemission, the 3d excited finalstates can only be reached via shake-up or by configuration interaction with

the 4s states, whereas in the shakedown process, both the 4s and the 3d fi-nal states are excited via the shakedown of the 4p electron) changes the ratiobetween the 3d and 4s excited states drastically. Figure 6.15(b) shows the 3pspectrum from the laser excited initial state. The 3d final states are much more

68

likely to be populated in the excited case. By studying the variations in inten-

sity ratio between features from the same final states in direct photoemission

and from shakedown, it was thus possible to quantify which final states that

are likely to have an electron in the 3d orbital, and which final states havean electron in the 4s orbital. The shake-down structure of potassium is con-

siderably much more complex than that of sodium, which is not surprising

considering the larger number of electrons.

6.3.1 Conclusions

The combination of synchrotron and laser radiation has been utilized to

perform spectroscopy on excited metal atoms. This excitation and probing

scheme allowed for observation of the shake-down process, which is

observable only from excited states. The difference in decay pathway

between the single photoionization and the shake-down process allows

characterization of the final states in a complex system such as potassium,

even though the final states in the two cases are the same. For sodium, it was

seen that an aligned ensemble of excited atoms were created during the laser

excitation, through performing a study of the linear dichroism. These results

provide a good basis from which it is possible to begin to interpret other

laser-induced effects in similar spectra.

69

7. Future outlook

The synchrotron has facilitiated an almost exponential growth of knowledge in

the surface science community. New facilities and lightsources will continue

to open up new possibilities for new types of experiments and for experiments

on systems that have, hithterto, been impossible to investigate for various rea-

sons. The type of experiments that have been performed in this thesis will

benefit greatly from these developments, since today, they are really touching

on the limits of what is possible.

The combination of synchrotron radiation and lasers is especially promis-

ing, because of the unique ability to study the core electron structure of sys-

tems that are involved in a lot of processes in nature. A few such very in-

teresting prospects are the studies of atmospheric photochemistry, chemical

catalysis and various biological systems. The knowledge about such systems

today is very limited, and there are almost endless possibilities to imagine new

ways of studying them in the future.

The continued development of theory goes hand-in-hand with the experi-

ments, and is progressing at a quicker pace than ever before; on one hand,

computers are getting faster and faster, allowing for studies of larger systems

with “brute force” methods, on the other hand, very extensive work is being

performed in the areas of paralellization and algorithmic science, speeding

up computations by orders of magnitude in the best cases. Quantum molec-

ular dynamics is also becoming more usable because of these facts, which is

especially useful for systems such as clusters.

The interdisciplinary aspect of work like this is also highly fruitful, bring-

ing together the communities of biology, chemistry and physics to tackle ques-

tions that span these subject matters. As for the close future, the cluster studies

at the Uppsala group serve not only as interesting systems in themselves, but

also as finite model systems for the development of an understanding of pro-

cesses that take place in liquid systems; a key point if biologically important

systems are ever to be studied by techniques like the ones employed in this

thesis. It is the belief of many, including myself, that atomic, molecular and

cluster science has a very bright future, considering the need for minaturiza-

tion and development occuring in the nano-science and technology areas.

71

8. Acknowledgments

The number of people involved in bringing this thesis work to its conclusion is

large, and I thank each and everyone who has, in some way, contributed to it.

There are, of course, people who deserve special mention. First and foremost, I

would like to thank my supervisors, Olle, Gunnar andMaxim for always being

available to answer questions about all things practical and for all their encour-

agement over the years. I would also like to thank Svante, for his unflagging

confidence in his graduate students and for all our discussions. Joachim, thank

you for the laser work, it was a pleasure even if it was sometimes frustrating.

All of my fellow Ph. D. students, who provided fun and joy and a positive

atmosphere everyday at work, thank you. Especially, Andreas, with whom I

spent most of my time, writing, teaching, traveling and discussing. I would

like to thank the Finnish group from Oulu, and the Bergen group, for allowing

me to join in their beamtimes, and for allowing me to visit their universities

for experiments and course work. All of the people at Fysikum, for providing

an open and productive working environment, thank you! Also, thanks to the

MAX-lab staff, who provided the most odd bits and pieces for the experiments

to run smoothly.

I would also like to thank all of my friends, none mentioned, none forgotten,

for making living life a joy each day and for always being there when there

was a rough patch. Wherever and whenever you are around, there is always

fun to be had. All of the people working on a non-profit basis, producing the

fantastic stage shows known as spex, in which I have taken part over the years,

thank you. All members of HKF, who are just as deranged as one should be,

and who keep coming up with the most brilliant get-rich-quick-schemes all

the time. How come we’re not rich yet?

Finally, I’d like to thank my family, Mamma, Pappa, Mia and Sanna, for

being who you are, and for always encouraging me to do what I wanted to do.

73

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