Arakawa Inv 09

7/30/2019 Arakawa Inv 09

1/56


2/56

Introduction

Cryopumping: chemically clean and dust-free

promising means to achieve XHV (< 10-8 Pa)

H2 is the main target in UHV and XHV.

Pumping characteristics are determined mainly by thermal process

also by nonthermal process (DIET, ESD, PSD)


3/56

Neither thermal nor nonthermal process of physisorbed

H2 is fully understood, especially in XHV.

As for the thermal property, an adsorption isotherm

must be the most fundamental one, but we have no

precise data of H2 in UHV or at low coverage ( < 1) in

other word.


4/56

Some basic studies on H2 cryopumping

Isotherm H2 / SUS

Tdependence ofp ?

Effect of radiation ?

< 1 ?

C. Benvenuti et al., JVST (1976)


5/56

Isotherm H2/CO2 frost

< 0.3

?

I. Arakawa et al., JVST (1986)


6/56

< 1

?

H2 on

E. Wallen, JVST A (1997)


7/56

H2 on various surfaces

G. Moulard et al., Vacuum (2001)

< 1 ?


8/56

We have no precise adsorption isotherm of hydrogen

at low temperatures T< 10 K,

low pressures p < 10-7 Pa,

low coverages < 1.

This is mainly because of difficulties in

obtaining and keeping XHV conditions

and

determining the amount of hydrogen physisorbed.


9/56

To make it clear the adsorption isotherm of hydrogenat temperatures 10 K > T> 3 K,

pressures 10-7 Pa >p > 10-12 Pa,

coverages

1 > > 0.0001. To make it clear the effect of nonthermal desorption

processes, DIET or ESD and PSD.

For this purpose, we utilize ESD as a probe of

physisorbed species.

Target of our present study


10/56

My talk in this workshop

I would like to introduce my studies on physisorptionof H2, He, and other rare gases.

In our studies, nonthermal desorption processes play

negative role: perturbation on thermal equilibrium,

positive role: useful means to probe adsorbate.

Short review of my early and recent works, and the

problems and questions encountered.


11/56

Four topics

(1)

Growth and structure of rare gas films on metal singlecrystal: application of LEED.

(2)Adsorption isotherm of H2 on metal surface:

application of ESD as a probe of physisorbed H2.

(3)Dubinin-Radushkevich isotherm of He on frost of gas

condensate as an adsorbent.

(4)Temperature dependence of equilibrium pressure of

physisorbed hydrogen.


12/56

1) Growth and structure of rare gas films on metal single

crystal: application of LEED.


13/56


14/56

LEED and ellipsometry apparatus

gas inlet

system

laser PMT

polarizer analyzercompensator

FC1 FC2

lens

diaphragmpump

TMP

TMP

MV

XLEED

0 10 20[cm]

vacuum

gauge

ion gun


15/56

LEED and ellipsometric isobar: Ar/Ag(111)

(a) Ag(111) (b) monolayer (c) bilayer (d) ~100 layers2

1

0[

deg.]

45 40 35 30

Temperature [K]

pAr = 8.3 10- 6

Pa


16/56

Xe / Ag (111)

Ellipsometric (a) isotherm and (b) isobar.

Shrink of intermolecular distance dof Xe lattice.

d0: lattice constant of bulk

S. Igarashi et al., Langmuir (2003)


17/56

Effect of ESD on thermal equilibrium

ie : electron flux [e-

/m2

s]QESD : ESD rate [1/m2 s]

QTD : thermal desorption rate [1/m2 s]

QAD : adsorption rate [1/m2

s]

: adsorption density [1/m2]

Criterion for no or negligible

perturbation caused by ESD :QESD


18/56

ie [e-/m2 s] or [A/m2]

QESD [1/m2 s]

QTD [1/m2 s]

QAD [1/m2 s]

[1/m2]

In thermal equilibirium

QTD= Q

AD=

1

4

n v =p

eq

2mkT: condensation

coefficient

ESD yield

Q

ESD=i

ea

ESD

aESD: ESD cross section [m2]


19/56

rough, but severe estimation

= 1

peq = 10-8 Pa

m = 40 amu

T= 10 K

aESD = 1x10-19 m2

= 1x1019 1/m2

QESD


20/56

eXtremely low current LEED: XLEED

Electrons are pulse-detected by a position sensitive

detector (resistive anode) with channel plates and are

digitally accumulated by PC.

Ie = 1 pA

6x10

6

e

-

/s About 1 % of incident electrons are elastically

diffracted and contribute to the diffraction pattern.

10

6

- 10

7

dots (e

-

counts) are necessary for LEEDpattern analysis.

A few minutes of accumulation time.


21/56

Summary and questions (1)

No noticeable effect of ESD in our LEED study.

Electron spectroscopy in physisorption study at low

equilibrium pressures ( < 10-6 Pa)

even in a very severe case ie 1 pA/mm2 should be all right (?)

but in most cases

ie

1 nA/mm

2

would be acceptable (?)


22/56

2)Adsorption isotherm of H2 on metal surface:

application of ESD as a probe of physisorbed H2.


23/56

To make it clear the adsorption isotherm of hydrogen

at temperatures: 10 K > T> 3 K,

pressures: 10-7 Pa >p > 10-12 Pa,

coverages:

1 > > 0.0001. To apply ESD technique to determine H2 density.

With naive assumption that the cross section for H+

ESD is constant at < 1, and therefore H+ ESD

yield should be proportional to H2 coverage.

Target of H2/metal study


24/56

Experimental apparatus and method

BASE PRESSURE

3x10-9 Pa (N2 eq.)

SAMPLE

copper (OFHC) on

a sapphire rod

Ts = 5.9 - 6.5 K

( 1 K)


25/56

Time-of-Flight measurement of desorbed ions

Flight Length

300 mm

Electron Beam

Ee = 50 - 150 eV

Ie 1 A

pulse width

0.25 s interval = 0.2 ms

0.3 nA/mm2

MCP detector

pulsed electron gun

Cu substrate


26/56

TOF mass spectra of hydrogen ions

TOF mass spectra of

ESD ions from the H2

on the cold Cu surface

under the equilibrium

with gas phase at the

pressure ofp(H2).

Ts

6.5 K Ee = 150 eV

E

SD

ionintensity[a

rbitraryunits]

flight time [s]

H+

H+

H2+

H3+

photon

p(H2)=1.5x10-8 Pa

p(H2)=1.5x10-5 Pa


27/56

p(H2) dependence of H+, H2+, H3+ ESD yields

H+

H2+

H3+

Ts 6.5 K

Ee = 150 eV

p H2

( ) =proom

TS

Troom


28/56

Detailed discussion on this result will be presented

in EVC-11, Salamanca.

Physisorption Characteristics of Hydrogen in

Extermely High Vacuum: an Application of ESD

Technique to Probe Physisorbed Hydrogen

I. Arakawa, T. Kawarabuki, and T. Miura

A3.TM.OR.11, 12:00-12:20, Sept. 21, Tuesday


29/56

e- beam flux probing H2 : 0.3 nA/mm2 OK?

Effect of ESD in H2 case

At 1

= 1

peq = 2x10-6 Pa

m = 2 amuT= 6.5 K

aESD = 1x10-19 m2

= 1x1019 1/m2

ie


30/56

e- beam flux probing H2 : 0.3 nA/mm2 OK?

Effect of ESD in H2 case

At 1

= 1

peq = 2x10-6 Pa

m = 2 amuT= 6.5 K

aESD = 1x10-19 m2

= 1x1019 1/m2

ie


31/56

Ion desorption cross section

Rough estimation of the cross section for ion desorption

at monolayer coverage.

flight time [s]

H+

H2+ H3+

ESDi

onyi

eld

(a.u.)

Ts 6.5 K,p(H2)=1.5x10-5 Pa,

1, Ee = 150 eV

aion at = 1

H+ :!! 3x10-23 m2H2+ :! 1x10-24 m2H3+ :! 1x10-24 m2


32/56

aion at = 1

H+

:!! 3x10-23 m2H2+ :! 1x10-24 m2H3+ :! 1x10-24 m2

H+

H2+

H3+

The cross sections for H2+ and H2+ seems to be not constant

but to depend strongly on . This is certainly because of the

second order desorption kinetics: H+ + H2 H2+ or H3+.


33/56

How about the cross section for total desorption of H2

by electron or photon ?


I. Arakawa and Y. Tuzi, J. Nucl. Mater. (1984)

R. Calder et al., JVST A (1996)

V. Baglin et al., Vacuum (2002)

Substrate conditions strongly influence the desorption

yield.

No reliable data at coverages < 1


34/56


Adsorption isotherm of H2/copper at

T 6.5 K,

3x10-6 Pa >p > 10-8 Pa,

1 > > 0.001.

How reliable is the ESD isotherm ?

ESD cross section of physisorbed H2 at < 1

H+

:

3x10-23

m2

total : ?


35/56

3) Dubinin-Radushkevich isotherm of He on frost of gas

condensate as an adsorbent.


36/56

C : mol ratio of He to CO2

Tf : formation temperature of frost

Isotherm of He/CO2 frost

Ts = 7.3 K

I. Arakawa et al., JVST (1979)


37/56

Dubinin-Raduschkevich plot of He/CO2

Not on a straight line.

This deviation might

have universally been

observed in many

works but most of them

were not published.


38/56

D-R plot of He by Wallen

Ts = 4.2 K

He on



39/56

Ts dependence of inflexion point of DR plot


Ts =


40/56

At the inflection point, EcEad.

Ec =kTln(p0/p) :

energy to compress the gas to the saturation vapor pressure

Ead : isosteric heat of adsorption

According to Wallen:

above inflection (Ec < Ead) : follow DR isotherm

below inflection (Ec > Ead) : follow Freundlich isotherm

OK. But, why?

What does it mean?


41/56

Another possibility:

PSD of He, probably, by visible or infrared light.

ln = ln0 B kT

lnp +C

p0

2

C : parameter depending on PSD cross section,

pumping speed of a system, etc.

Assuming YPSD, DR equation can be modified as


42/56

Arakawas data and the modified equation

Its nice !

However, it does not

reproduce the Tdependence of the

inflection points

reported by Dr Wallen.


43/56

Helium does not follow the DR isotherm.

Why ?

Radiation ?

DR plot shows inflection points where EcEad.

Why ?

How can we explain Wallens suggestion ?



44/56

4) Temperature dependence of equilibrium pressure of

physisorbed hydrogen.


45/56

There are many papers reporting H2 isotherms,

from which we can find an abnormal temperature

dependence of equilibrium pressure of H2. #

However, there are few papers discussing about this

abnormality.


46/56

isoster


isothermeffect of radiation

H2/SUS


47/56

H2/CO2 frost

I.Arakawa and Y. Tuzi,

JVST A (1986)

high

Tslow

Diffusion into the frost layer

should be the rate

determining factor of

pumping speed.

the higher Tsthe lowerp(H2) !?


48/56

H2/Xe frost

isotherm

isoster

I. Arakawa, JVST A (1986)


49/56

isosterisoster

H2/Xe by Arakawa

H2/SUS by Benvenuti

Radiation ?

Is that all ?


50/56

H2/Xe frost

A coincidence of

Ts dependence

between ESD

yield andp(H2).

Any change of

adsorptioncondition ?

ESD yield

isoster

I. Arakawa, JVST A (1986)


51/56

Abnormal temperature dependence of physisorbed

hydrogen.

What is its origin ?

Radiation effect ?

Diffusion process ?

Change of adsorption condition ?



52/56

Summary

(1) Effect of ESD in the LEED study of rare gas films.

(2) Application of ESD as a probe of physisorbed H2.

(3) Deviation of the He isotherm from DR isotherm.

(4) Abnormal temperature dependence of H2 isotherm.

Thank you for your attention.


53/56

(1) Effect of ESD in the LEED study of rare gas films.

No noticeable effect of ESD in our XLEED study.

Electron spectroscopy in physisorption study at low

equilibrium pressures ( < 10-6 Pa)

even in a very severe case

ie 1 pA/mm2 should be all right (?)

but in most cases

ie 1 nA/mm2 would be acceptable (?)


54/56

(2) Application of ESD as a probe of physisorbed H2.

How reliable is the ESD isotherm ?

Adsorption isotherm of H2/copper at

T 6.5 K,

3x10-6 Pa >p > 10-8 Pa,

1 > > 0.001.

ESD cross section of physisorbed H2 at < 1

H+

:

3x10-23

m2

(reasonable ?) total : ?


55/56

(3) Deviation of the He isotherm from DR isotherm.

He does not follow the DR isotherm.

Why ?

Radiation ?

Inflexion point, where EcEad.

Why ?

What does it mean ?


56/56

(4) Adsorption isotherm of submonolayer H2.

Abnormal temperature dependence of physisorbed

hydrogen.

What is its origin ?

Radiation ?

Diffusion ?

Change of adsorption state ?

Documents

Arakawa Inv 09