surface science
ELSEVIER
Surface Science 390 (1997) 146 151
Electron-stimulated desorption of potassium and cesium atoms
from oxidized molybdenum surfaces
V . N . A g e e v a,,, Y u . A . K u z n e t s o v a, T . E . M a d e y b
a A.F. Ioffe Physico-Technical Institute, Academy of Sciences of Russia, 194021 St Petersburg, Russia
b Department of Physics and Astronomy, Laboratory for Surface Modification, Rutgers, The State University of New Jersey,
Piscataway, NJ 08855, USA
Received 15 February 1997; accepted for publication 26 June 1997
Abstract
The electron-stimulated desorption (ESD) yields and energy distributions for potassium (K) and cesium (Cs) atoms have been
measured from K and Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. The
measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appearance threshold for K
and Cs atoms is independent of the molybdenum oxidation state and is close to the oxygen 2s level ionization energy of 25 eV.
Additional thresholds for both K and Cs atoms are observed at about 40 and 70 eV in ESD from layers adsorbed on an oxygen
monolayer-covered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD
energy distributions for K and Cs atoms consist of single peaks. The most probable kinetic energy of atoms decreases in going from
cesium to potassium and with increasing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs
atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward very low
kinetic energies. The data can be interpreted by means of the Auger stimulated desorption model, in which neutralization of adsorbed
alkali-metal ions occurs after filling of holes created by incident electrons in the O 2s, Mo 4s or Mo 4p levels. © 1997 Elsevier
Science B.V.
Keywords: Alkali metals; Atom emission; Electron stimulated desorption (ESD); Molybdenum oxides
1. Introduction
Electron-stimulated desorption (ESD) is successfully used to study and modify adsorbed layers.
However, there are outstanding issues concerning
the mechanisms for ESD of adsorbed species.
Whereas desorption of ions has been well studied
by many investigators, there is a lack of direct
simultaneous measurements of desorption yields
and energy distributions for neutrals [ 1]. We have
* Corresponding author. E-mail: [email protected]
0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.
PII S0039-6028 (97) 00532-3
carried out detailed measurements of ESD crosssections and energy distributions for alkali metal
[2,3] and barium [4] atoms from layers adsorbed
on oxidized tungsten over a wide temperature and
metal-coverage range using a surface ionization
detector [5]. It has been found that ESD of neutral
atoms can be induced by the ionization of adparticle and substrate oxygen core levels, the excitation channel being dependent on the adparticle
localization on the surface with the respect to the
substrate atoms. This paper reports on the ESD
yields and energy distributions for K and Cs atoms
from layers adsorbed on oxidized molybdenum.
V.N. Ageev et aL / Surface Science 390 (1997) 146-151
147
The results are compared with recent measurements for ESD of alkali and Ba atoms on oxidized
tungsten [2-4].
current density does not exceed 1 0 - 6 A / c m 2 at
electron energy Ee = 100 eV, and there is no noticeable heating of the sample.
2. Experimental
3. Results
The experimental set-up and the procedures used
in preparing the sample for measurements are
described in detail elsewhere [6]. In short, the
experiments are performed in an ultrahigh vacuum
chamber with a base pressure of about
5 x 10-1°Torr. A textured m o l y b d e n u m ribbon
measuring 70 x 2 x 0.01 m m is used as the sample.
To produce a predominantly (100) oriented surface
the ribbon is heated in ultra-high vacuum at
2000 K for 5 h by alternating current [7]. The
sample is cleaned of carbon by annealing in an
oxygen atmosphere [ P ( O 2 ) = l x l 0 - 6 T o r r ]
at
1800 K for 3 h. After pumping oxygen from the
chamber, the sample is heated at 2200 K for 3 min
to desorb the oxygen. Two sample preparations
are used for the ESD experiments. An oxygen
monolayer is prepared by exposing the sample
(T=1400K)
to oxygen at a pressure of
1 x 1 0 - 6 T o r r for 10s and a thick molybenum
oxide film is prepared by heating the sample at
1000 K in oxygen at 1 x 10 .6 Torr [8].
Alkali-metal atoms are deposited onto the oxidized m o l y b d e n u m surface at 300 K from directly
heated evaporators by thermal decomposition of
the corresponding chromates. The concentration
of deposited metal is determined from the deposition time under constant alkali-metal flux. The
alkali-metal flux intensity is measured by means
of the total alkali-metal current during surface
ionization at the sample surface heated to the
current saturation temperature (T_> 1750 K).
The concentration of adsorbed K and Cs
atoms corresponding to one monolayer on the
oxidized Mo(100)
face is taken to be
N = 5 × 1014 atoms/cm 2.
The ESD energy distributions for alkali-metal
atoms are measured by means of a time-of-flight
method by pulsing the electron beam. The
desorbed alkali-metal atoms are ionized in a surface ionization detector consisting of a textured
tungsten ribbon heated to T = 1500 K. The electron
Electron b o m b a r d m e n t of K or Cs films
adsorbed at T = 300 K on an oxidized molybdenum
surface leads to desorption of K and Cs atoms.
The ESD dependences for yields and thresholds of
K and Cs atoms are similar, so we describe mainly
those for Cs atoms. Fig. 1 shows a plot of the Cs
atom ESD yield, q, from a layer adsorbed on an
oxygen-monolayer-covered molybdenum surface
(curve 1) and on a molybdenum oxide surface
(curve 2) as a function of electron energy Ee. One
can see the same Cs a t o m appearance threshold in
ESD from the m o l y b d e n u m surfaces with different
degrees of oxidation, a value ( ~ 2 5 eV) which is
close to the ionization energy of the oxygen 2s
level [9] taking into account the contact potential
difference between the electron emitter and sample.
Additional thresholds for Cs atoms are distinctly
observed at about 40 and 70 eV, but only from a
layer adsorbed on an oxygen-monolayer-covered
molybdenum surface (Fig. 1, curve 1); the additional thresholds are not seen for ESD of Cs from
the oxidized Mo surface (Fig. 1, curve 2). The
values of the additional thresholds correlate well
with the ionization energies of the molybdenum
4
.•
,
i
,
~
,
i
.
L
,
~
,
i
,
1
3
E
o
2
b
3
2
r --
0
0
20
40
60
80
E e
,
100
120
140
160
eV
Fig. 1. Cs atom ESD yield q as a function of electron energy
E~ from an adlayer on an oxygen-monolayer-covered molybdenum surface (curve 1), on a molybdenum oxide surface (curve
2) and on a tungsten oxide surface (curve 3).
148
V.N. Ageev et aL / Surface Science 390 (1997) 146-151
4p a n d 4s levels, respectively [9]. T h e Cs a t o m
E S D yield f r o m a n a d l a y e r on a n o x i d i z e d m o l y b d e n u m surface reaches no s a t u r a t i o n as the
electron energy increases u p to 500 eV (curves 1
a n d 2).
T h e E S D o f Cs f r o m o x y g e n - c o v e r e d M o can
be c o m p a r e d with E S D o f Cs f r o m o x y g e n - c o v e r e d
W . N o t e t h a t the q(Ee) d e p e n d e n c e in the E S D o f
Cs a t o m s f r o m oxidized t u n g s t e n does n o t reveal
s e c o n d a r y t h r e s h o l d s a n d s a t u r a t e s with increasing
electron energy E~ a b o v e 50 eV ( F i g . 1, curve 3)
[5]. I n a d d i t i o n , the E S D yield for Cs a t o m s does
n o t d e p e n d o n the degree o f o x i d a t i o n o f tungsten,
whereas the yield decreases b y a b o u t two times as
one goes f r o m a n o x y g e n - m o n o l a y e r - c o v e r e d
m o l y b d e n u m surface t o a t h i c k m o l y b d e n u m oxide
surface at the s a m e electron energy a n d cesium
coverage.
T h e electron energies for the a d d i t i o n a l thresholds are i n d e p e n d e n t o f t h e cesium c o v e r a g e
( F i g . 2). F o r electron energies in the r a n g e
4 0 - 1 0 0 eV, the Cs a t o m E S D yield increases linearly as the cesium c o v e r a g e increases a n d reaches
a m a x i m u m a r o u n d 0 = 0 . 9 to 1.0. A n a n a l o g o u s
q(O) d e p e n d e n c e is o b s e r v e d in the E S D o f Cs
a t o m s f r o m a layer on a n oxidized t u n g s t e n surface [51.
T h e E S D energy d i s t r i b u t i o n s for K a n d Cs
a t o m s f r o m layers on an o x i d i z e d m o l y b d e n u m
surface consist o f single p e a k s , their shapes a n d
p e a k p o s i t i o n s being i n d e p e n d e n t o f the electron
energy E~. E a c h energy d i s t r i b u t i o n b e c o m e s
e x t e n d e d t o w a r d s very low kinetic energies while
its h i g h - e n e r g y p a r t does n o t a p p r e c i a b l y change
as one goes f r o m a m o l y b d e n u m oxide surface to
a n o x y g e n - m o n o l a y e r - c o v e r e d m o l y b d e n u m surface ( F i g . 3). T h e Cs a t o m E S D energy distribution f r o m a layer a d s o r b e d o n an oxidized
m o l y b d e n u m surface shifts t o w a r d s low energies
w i t h o u t a c h a n g e in s h a p e with increasing cesium
c o v e r a g e ( F i g . 4).
I n c o m p a r i s o n , it s h o u l d be n o t e d t h a t the
degree o f o x i d a t i o n o f t u n g s t e n does n o t affect the
1,2
1,0
0,8
.
0,6
z
"o
0,4
6,2
•
o,
0,0
0,1
1
~.~<
2
0,2
0,3
0,4
0,5
E , eV
Fig. 3. Normalized ESD energy distributions for Cs atoms from
a layer adsorbed at T= 300 K on an oxygen-monolayer-covered
molybdenum surface (curve 1) and on molybdenum oxide surface (curve 2). Cesium coverage O=0.125. Electron energy
E~= 80 eV.
1,2
5
3
1,0
0,8
E
o
~2
2
0,6
b
~'~ 0,4
1
0,2
r
20
40
60
80
100
120
140
160
Ee , eV
0,6
0,0
I
0,1
,
0,2
0,3
0,4
0,5
E , eV
Fig. 2. Cs atom ESD yield q as a function of electron energy
E, from a layer adsorbed at T= 300 K on an oxygen-monolayercovered molybdenum surface at different cesium coverages. For
curves 1, 2 and 3, the cesium coverages O are 0.25, 0.5 and
0.75, respectively.
Fig. 4. Normalized Cs atom ESD energy distributions from a
layer adsorbed at T= 300 K on molybdenum oxide surface for
two cesium coverages: (1) 0.125 and (2) 0.75. Electron energy
E~= 80 eV.
V.N. Ageev et al. / Surface Science 390 (1997) 146-151
ESD energy distribution for Cs atoms from Cs
adsorbed on an oxidized tungsten surface [6].
However, an effect of cesium coverage on the Cs
a t o m ESD energy distribution is observed from a
layer on an oxidized tungsten surface, the shift
being approximately two times bigger than for
oxidized M o [6]. Although the ESD dependences
for Cs and K atoms from layers adsorbed on an
oxidized m o l y b d e n u m surface are quite similar
there are some small quantitative differences
between them: (1) the ESD yield for K atoms is
higher by about 20% than that for Cs atoms at
the same metal coverage and electron energy; (2)
the low-energy "tail" for a K a t o m ESD energy
distribution is somewhat shorter than that for Cs
atom ESD energy distribution.
4. Discussion
To interpret the data we use the Auger stimulated desorption model that was developed to
account for the ESD of alkali-metal atoms from
layers adsorbed on an oxidized tungsten surface
[10]. Recall that alkalis (Cs, K ) adsorbed on
oxidized M o are believed to be ionic in nature at
low coverages. According to the desorption model,
a vacancy created by the primary electron in the
O 2s level can be filled by an electron from the
O 2p level causing neutralization of an adsorbed
alkali-metal ion by an Auger electron from the
O 2p level. If the positive oxygen ion recovers its
negative charge by capturing electrons from the
substrate faster than the newly formed alkali-metal
a t o m becomes reionized, it starts repelling the
alkali-metal a t o m by overlapping their valence
orbitals and the a t o m can escape from the surface.
The additional desorption thresholds at ~ 4 0
and 70 eV imply that there exist other adsorption
bond excitation channels resulting in the ESD of
alkali-metal atoms from layers adsorbed on an
oxygen-monolayer-covered m o l y b d e n u m surface.
Particularly interesting is the shape of the ESD
yield curves, in which sharp m a x i m a are observed
at ~ 40 and 70 eV. These energies correspond to
the electron binding energies for the M o 4p and
4s levels, respectively. I f the thresholds corresponded simply to one-electron ionization processes, one would expect a rounded step-like
149
increase in Cs yield above threshold. However, the
existence of sharp peaks so close to the M o 4p,
Mo 4s thresholds seems to indicate that desorption
proceeds via a resonance process, in which the
initial excitation m a y lead to a spectator electron
in a quasi bound state, forming a core exciton or
inner well resonance.
Resonance processes are well known in ESD of
negative ions [11] and neutral species [12]. In a
study of electron-stimulated disordering of
O/Mo(011), Fedorus et al. [13] report a similar
sharp feature at ~ 65 eV, which they associate with
excitation of Mo 4s. They attribute the occurrence
of the sharp peak (rather than a step-like feature)
to a "peculiarity in the ionization probability of
the core level". They do not report any structure
associated with the M o 4p excitation. Photon stimulated desorption spectra of H +, F + and O ÷ from
several oxide surfaces have also revealed features
that have been associated both with core excitons
and with inner well resonances whose structure is
sensitive to the local environment [14,15].
T o account for the desorption of neutral Cs at
~ 4 0 and 70 eV, we need to understand how
adsorbed alkali ions are neutralized and how the
repulsive final state is formed. Holes created in
M o 4p and 4s levels can be filled by transitions
involving
O 2s
or
M o 4p,
respectively.
Neutralization of an adsorbed alkali-metal ion
m a y proceed by the capture of an electron released
in the Auger cascade processes involving M o 4s,
M o 4p, O 2s and O 2p. Alternatively, the spectator
electron formed in resonance excitation of M o 4p,
M o 4s may be captured by the alkali ion, or by a
neighboring oxygen ion. This mechanism manifests
itself only in ESD from an oxygen-monolayercovered molybdenum surface (Fig. 1, curve 1 ) and
is not seen in ESD from a n M o oxide surface
(Fig. 1, curve 2). This difference m a y be related
to the high probability for a positive oxygen ion
to recover its negative charge on the monolayercovered surface, which is not as likely on the
oxide surface.
N o secondary thresholds are found in ESD from
layers adsorbed on any of the oxygen-covered or
oxidized tungsten surfaces studied; this m a y be
due to the relatively low ionization cross-sections
by electrons for W 5s and W 5p levels as compared
150
V.N. Ageev et al. / Surface Science 390 (1997) 146 151
with these for Mo 4s and Mo 4p levels at the same
electron energy [t6] as well as to the low Auger
decay probability for core levels of tungsten in
comparison with that of molybdenum [9].
The extended low-energy "tails" in the ESD
energy distribution for K and Cs atoms from
layers adsorbed on an oxygen-monolayer-covered
molybdenum surface (Fig. 3) can be related to a
process involving reverse atom motion. A newly
formed alkali-metal atom can approach the surface
in the field of the positive oxygen ion. The closer
the atom comes to the surface before the oxygen
ion charge relaxation occurs, the higher the kinetic
energy it acquires. The relaxation time on an
oxygen-monolayer-covered molybdenum surface is
expected to be shorter than that on a molybdenum
oxide surface, thus implying that the atoms
desorbed from a layer on an oxygen-monolayercovered molybdenum surface should have a lower
kinetic energy. Apparently, the relaxation time on
an oxidized tungsten surface is sufficiently long
that the K and Cs atoms in ESD from layers
adsorbed on this surface obtain a higher kinetic
energy. This supposition is supported by the higher
Cs atom ESD yield from a layer on an oxidized
tungsten surface as compared with that on an
oxidized molybdenum surface at electron energy
Ee below 100 eV (Fig. 1). It should be mentioned
that the ESD yield for K atoms is slightly higher
than that for Cs atoms, while the contribution of
low energy K atoms to the ESD energy distribution
is somewhat lower than that for Cs atoms. The
former effect can be due to the higher ionization
potential for K atoms and, respectively, to their
lower ionization probability as compared with that
for Cs atoms [17]. Apparently, the latter effect can
be associated with the higher mass of Cs atoms,
which move a smaller distance toward the surface
than K atoms before the positive oxygen charge
relaxation occurs, even though they possess a
higher polarizability than K.
5. Summary
The ESD of K and Cs atoms is observed from
layers adsorbed at T = 3 0 0 K on an oxidized
molybdenum surface. Their appearance threshold
is independent of the molybdenum oxidation state
and is about 25 eV. However, additional thresholds
(at about 40 and 70 eV) and energy distributions
extended toward very low energies for both K and
Cs atoms are observed only from layers adsorbed
on an oxygen-monolayer-covered molybdenum
surface. The data can be interpreted by means of
the Auger-stimulated desorption model in which
neutralization of adsorbed alkali-metal ions occurs
after Auger decay of holes created by incident
electrons in the O 2s, Mo 4p and Mo 4s levels. The
special role of the oxygen-monolayer-covered
molybdenum surface is associated with the relaxation rate for the positive charge on oxygen ions
on this surface.
Acknowledgements
The authors thank Professor O.V. Konstantinov
for valuable discussions. This work was performed
within the framework of the Russian State
Program "Atomic Surface Structures", Project
95-1.27. The authors also acknowledge the Russian
Fundamental Research Foundation for supporting
this work through Grant 95-02-04081a, and one
of us (T.E.M.) acknowledges partial support by
N A S A Planetary Atmospheres Division.
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