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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)
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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.
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Some basic studies on H2 cryopumping
Isotherm H2 / SUS
Tdependence ofp ?
Effect of radiation ?
< 1 ?
C. Benvenuti et al., JVST (1976)
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Isotherm H2/CO2 frost
< 0.3
?
I. Arakawa et al., JVST (1986)
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< 1
?
H2 on
E. Wallen, JVST A (1997)
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H2 on various surfaces
G. Moulard et al., Vacuum (2001)
< 1 ?
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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.
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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
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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.
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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.
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1) Growth and structure of rare gas films on metal single
crystal: application of LEED.
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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
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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
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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)
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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
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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]
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rough, but severe estimation
= 1
peq = 10-8 Pa
m = 40 amu
T= 10 K
aESD = 1x10-19 m2
= 1x1019 1/m2
QESD
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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.
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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 (?)
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2)Adsorption isotherm of H2 on metal surface:
application of ESD as a probe of physisorbed H2.
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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
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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)
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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
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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
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p(H2) dependence of H+, H2+, H3+ ESD yields
H+
H2+
H3+
Ts 6.5 K
Ee = 150 eV
p H2
( ) =proom
TS
Troom
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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
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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
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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
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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
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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+.
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How about the cross section for total desorption of H2
by electron or photon ?
C. Benvenuti et al., JVST (1976)
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
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Summary and questions (2)
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 : ?
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3) Dubinin-Radushkevich isotherm of He on frost of gas
condensate as an adsorbent.
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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)
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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.
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D-R plot of He by Wallen
Ts = 4.2 K
He on
E. Wallen, JVST A (1997)
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Ts dependence of inflexion point of DR plot
E. Wallen, JVST A (1997)
Ts =
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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?
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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
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Arakawas data and the modified equation
Its nice !
However, it does not
reproduce the Tdependence of the
inflection points
reported by Dr Wallen.
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Helium does not follow the DR isotherm.
Why ?
Radiation ?
DR plot shows inflection points where EcEad.
Why ?
How can we explain Wallens suggestion ?
Summary and questions (3)
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4) Temperature dependence of equilibrium pressure of
physisorbed hydrogen.
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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.
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isoster
C. Benvenuti et al., JVST (1976)
isothermeffect of radiation
H2/SUS
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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) !?
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H2/Xe frost
isotherm
isoster
I. Arakawa, JVST A (1986)
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isosterisoster
H2/Xe by Arakawa
H2/SUS by Benvenuti
Radiation ?
Is that all ?
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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)
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Abnormal temperature dependence of physisorbed
hydrogen.
What is its origin ?
Radiation effect ?
Diffusion process ?
Change of adsorption condition ?
Summary and questions (4)
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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.
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(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 (?)
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(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 : ?
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(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 ?
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(4) Adsorption isotherm of submonolayer H2.
Abnormal temperature dependence of physisorbed
hydrogen.
What is its origin ?
Radiation ?
Diffusion ?
Change of adsorption state ?
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