First inelastic xray scattering spectroscopy measurements of the dynamicstructure factor S(q,ω) of electrons in a solid noble gas (He)W. Schülke, A. Kaprolat, K.J. Gabriel, N. Schell, E. Burkel, and R. O. Simmons Citation: Review of Scientific Instruments 66, 1578 (1995); doi: 10.1063/1.1145913 View online: http://dx.doi.org/10.1063/1.1145913 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/66/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Nuclear dynamics and spectator effects in resonant inelastic soft x-ray scattering of gas-phase watermolecules J. Chem. Phys. 136, 144311 (2012); 10.1063/1.3702644 Electronic structure of lithium battery interphase compounds: Comparison between inelastic x-rayscattering measurements and theory J. Chem. Phys. 135, 224513 (2011); 10.1063/1.3664620 Large Solid Angle Spectrometer for Inelastic Xray Scattering AIP Conf. Proc. 879, 1837 (2007); 10.1063/1.2436428 Static structure factor and electron correlation effects studied by inelastic x-ray scattering spectroscopy J. Chem. Phys. 108, 4545 (1998); 10.1063/1.475865 Electronic structure of Al and Si dissolved in transition or noble metals studied by soft xray spectroscopy Appl. Phys. Lett. 27, 529 (1975); 10.1063/1.88295

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First inelastic x-ray scattering spectroscopy measurements of the dynamic structure factor S(q, 01) of electrons in a solid noble gas (He)

W. Schijlke, A. Kaprolat, and K.J. Gabriel Institute of Physics, University of Dortmund, D-44221 Dortmwui, Germany

N. Schell and E. Burke1 Institute of Applied Physics, University of Erlangen, D-91054 Erlangen, Germany

FL 0. Simmons University of Illinois, Urbana, Illinois 61801

(Presented on 18 July 1994)

Inelastic x-ray scattering spectroscopy measurements on hcp single crystal solid He with qllc axis and q=O.45, 0.97, and 1.24 a.u. are presented. The crystals were grown in situ under a pressure of 600 bar at 5-7 K within a Be hollow cylinder. By subtracting a measurement on the empty Be cylinder, pure solid He spectra are obtained and discussed in terms both of the hcp He band structure and of excitonic excitation. The existence of an exciton at 21.8 eV above the 1s core level could unambiguously be settled. Clear evidence is found that the bottom of the conduction band is at 27 eV. 0 1995 American Institute of Physics.

I. INTRODUCTION

The investigation of the electronic structure and of elec- tronic excitations in solid noble gases by means of VUV and soft-x-ray spectroscopy was one of the first domains of syn- chrotron radiation research in the early 1970s (see Ref. 1 for a review). Only solid He could not be used, to the knowledge of the authors, for these kinds of investigations, since the solid phase needs pressure in the kbar regime and tempera- tures of a few kelvins. High-pressure cells are therefore nec- essary, so that an appropriate spectroscopy must enable transmission of the pressure cell walls. Thus inelastic x-ray scattering spectroscopy (IXSS) studies of electronic excitation,2 making use of the high penetration power of hard x rays, combined with spectroscopy in the eV range3 seems to be the best choice. In order to demonstrate the feasibility of those experiments we present the results of IXSS mea- surements on solid He. Section II refers to details of the instrument and modes of the measurements. Section III pre-

sents the experimental results, which are discussed in terms both of the conduction band structure of solid He, and of possible near-threshold excitonic excitations.

II. INSTRUMENTATlON AND MEASUREMENT

The IXSS measurements where performed using the in- strumentation of the HARWI (Harter Riintgen-wiggler) In- elastic Scattering Beam line (HISB), described in detail elsewhere.4 The whole setup of this beam line is sketched in Fig. 1 and consists of a fixed-exit Si(511) double-crystal monochromator, where the first (plane) crystal is connected with a water-cooled copper block by liquid gallium, and the second (segmented) crystal is cylindrically bent in order to achieve sagittal focusing to the sample position. Energy analyzing is performed in inverse geometry by means of a spherically bent Si(12 0 0) crystal operating at a Bragg angle of 86”, where the transmitted pass band was 1.6 eV

PMC = plane monochromalor crystal SFMC = aagittnlly focusing monocbromator crys1.4

ss = scatlering sample SBAC = spherically bent analyzer cryslal

SSD = solid stntc dclector

FIG. 1. Setup of the HARWI inelastic scattering beam line at HASYLAE3.

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;;;5.0 2

54.0

e s3.0

h 5 2.0

g 2 1.0 Y

0.0 50 “sb

FIG. 2. IXSS spectra for q=O.97 au., q/c axis of the He single crystal, normalized in such a way that the difference of both spectra is, on the average, kept zero between 4 and 15 eV. Solid line: spectrum of the Be hollow cylinder filled with single crystal solid He; dashed line: spectrum of the empty Be cylinder.

wide and situated at 13.7 keV. The whole analyzer can be rotated around the sample, in order to offer a wide range of scattering angles.

The solid He crystal has been prepared in situ within the scattering chamber of the HISB. The growth of the hcp crys- tal occurs in a Be hollow cylinder, connected with a pressure-generating cell and cooled down to 5-7 K by means of a He cryostat. The pressure-generating cell (PGC) is filled with He gas under a pressure of 200 bar when bathing in liquid nitrogen. By elevating the temperature of the PGC to room temperature the pressure in the Be cylinder is boosted, so that by repeating this procedure two or three times, one can reach the final pressure of 600 bar, sufficient to grow a hcp crystal, the orientation of which is controlled by x-ray diffraction.

The IXSS spectra have been measured with qjlc axis and for three different values of the momentum transfer 4:

o’6 r-----r--- -

-rl-ry-i--r-rT’r,a,,,.,, ,,,,,I III 60 “;

FIG. 3. IXSS difference spectrum for q=O.45 au.: empty-cell spectrum subtracted from the filled-cell spectrum.

-0.4 LTTTmrrrrrrTmrrrrrrTn 0 20

Ener2; (eV) 60 -7

FIG. 4. The same as Fig. 3, but for q=O.97 a.“.

q=O.45, 0.97, and 1.24 a.u., in order to see eventual disper- sion of spectral line structure. We collected between 7000 and 15 000 counts in each measuring point near the peak of the inelastic part of the spectrum.

In order to obtain the pure IXSS spectrum of solid Hel one has to subtract the contribution of the Be cylinder and possibly of the aluminized mylar foils of the cryostat. This could be achieved by measuring in each case a spectrum of the “empty” Be cylinder (filled only with He gas of 1 bar), obtained under conditions identical to those chosen for the filled cylinder, and by subtracting the empty-cell measure- ment from the filled-cell measurement, where both spectra are normalized in such a way that, for physical reasons, the difference of both spectra is, on the average, kept zero be- tween 4 and 15 eV energy transfer.

HI. RESULTS AND DlSCUSSlON

Figure 2 presents an “empty” and a “filled” spectrum for q=O.97 a.u., after background subtraction and removal of the quasielastic line (this removal leads to uncertainty be- tween 0 and 4 eV, a range which is therefore not plotted in

0.6

- 0.6 47 .d s 0.4

d 2 0.2

g -0.0

? 2 -0.2 E

-0.4 ,IIIIL(~*II~1~I1~I~(

20 Ener$J (eV)

fsT---

FIG. 5. The same as Fig. 3, but for q=l.24 a.u.

I

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Fig. 1). Since the exact beam profile of the incident beam is not known, one can only estimate that the contribution of solid He to the “filled” spectrum must be around 10% or less, due to the small density of solid He compared with the density of Be. Therefore, it is not possible to normalize the spectra according to the f-sum rule, the method which we normally use to get absolute values of the dynamic structure factor S(q,o) from measured IXSS spectra? The normaliza- tion procedure described in Sec. II can only provide S(q,o) in arbitrary units.

The most prominent difference between both spectra is around 20 eV. The difference spectra for the three q values are plotted in Figs. 3, 4, and 5, respectively. One prominent structure (screened) can be seen in all difference spectra, namely a sharp peak, lying, within experimental error, for all q’s at 21.820.3 eV. The width of this peak seems to be resolution limited. We attribute this peak to an exciton (or a very narrow-lying exciton series), as has already been found in the excitation spectra of other solid noble gases, for in- stance, in Ar.r A second structure is also visible in all three difference spectra; this is the steep rise at 27 eV, which we associate with the bottom of the He conduction band. Band

structure calculations6 found the bottom of the conduction band 30 eV above the 1s core level. We think that our ex- periments give strong evidence for a lower position of this band by 3 eV. All further fine structure of the spectra is more or less within the statistical error bars, with one exception: the strong dip near 50 eV in the q=O.97 difference spectrum. So far we have no explanation for that structure.

ACKNOWLEDGMENT

This work has been funded by the German Federal Min- istry of Research and Technology under Contract No. 05 SPEAXB 6.

‘B. Sonntag, in Rare Gas Solids, edited by M. L. Klein and J. A. Venables (Academic, New York, 1976), Vol. 11, Chap. 17.

‘W. Schiilke, in Handbook on Synchrotron Radiation, edited by G. Brown and D. E. Moncton (Elsevier, Amsterdam, 1991), Vol. 3, pp. 565-637.

“H. Nagasawa, S. Mot&is, and W. Schiilke, J. Phys. Sot. Jpn. 58, 710 (1989).

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5 W. Schiilke, H. Nagasawa, S. Mourikis, and P. Lanzki, Phys. Rev. B 33, 6744 (1986).

‘R. 0. Simmons (private communication).

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