Surface Science 469 (2000) 71–79
www.elsevier.nl/locate/susc
Formation of autoionizing Ne11 in grazing collisions
with an Al(111) surface
O. Grizzi a, *, E.A. Sánchez a, J.E. Gayone a, L. Guillemot b, V.A. Esaulov b,
R.A. Baragiola c
a Centro Atómico Bariloche, Instituto Balseiro, CNEA, and CONICET, 8400 Bariloche, Argentina
b Laboratoire des Collisions Atomiques et Moléculaires, (Unité Mixte de Recherche CNRS–Université No. 8625), Bâtiment 351,
Université Paris-Sud, F-91405 Orsay, France
c Engineering Physics, University of Virginia, Charlottesville, VA 22904, USA
Received 15 May 2000; accepted for publication 18 August 2000
Abstract
We present a study of the production of neon autoionizing states during glancing scattering of 3–50 keV Ne+ on
an Al(111) surface of varying degree of roughness. The projectile energy dependence of the intensity of neon
autoionization peaks in the electron spectra indicates that the various autoionizing states fall into two main groups.
At low keV energies, Ne(2p43s2) autoionizing states are formed, while at higher energies other peaks appear in the
electron spectra with similar intensity. Some of these peaks may be due to low-lying 2p4nln∞l∞ states, whose population
is aided by the motion of the projectile. Other peaks can be assigned to 2s2p5nl, 2s2p6nl and 2p3nln∞l∞ states. A study
of the effect of surface roughness shows that the production of excited states in grazing scattering is favored at
rougher surfaces. For flat surfaces the peaks become broader and less intense, an effect that we ascribe to autoionization
decay closer to the surface, based on previous numerical simulations of peak shapes. © 2000 Elsevier Science B.V.
All rights reserved.
Keywords: Aluminum; Electron emission; Ion scattering spectroscopy; Ion–solid interactions; Noble gases
1. Introduction
Projectile excitation in collisions of neon atoms
and ions with surfaces has been studied extensively
over the past 25 years [1–31] because it provides
one of the richest and best-characterized examples
of inelastic ion–surface collisions. In particular,
several studies have been made on the production
of autoionizing states of neon in collisions with
magnesium, aluminum and silicon surfaces, which
* Corresponding author. Fax: +54-2944-445299.
E-mail address: [email protected] (O. Grizzi)
are observed by spectroscopy of the emitted
electrons. Most of these studies were performed
on polycrystalline surfaces and for projectiles in
the low keV energy range. Strong differences were
observed with gas-phase collision studies [32–35]
concerning the type of excited states produced.
Unlike gas-phase collisions, where a multitude of
autoionization peaks is observed in the electron
spectra, in ion–surface collisions only two major
peaks are observed, attributed to the decay of
Ne11[2p4(3P)3s2] and [2p4(1D)3s2] (peaks labeled I
and II, respectively, in Fig. 1) [6,7]. Also, ion–
surface collisions experiments show the predomi-
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0 0 39 - 6 0 28 ( 00 ) 0 08 0 5 -0
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O. Grizzi et al. / Surface Science 469 (2000) 71–79
Fig. 1. (a) Electron energy spectrum for 3 keV Ne+–Al (111)
collisions at 6° incidence. Inset: schematic geometry showing
the projectile incident angle (a) and the direction of electron
observation characterized by the angles (h, w) measured from
the ion direction and the scattering plane, respectively. (b)
Electron energy spectrum for 20 keV Ne+–Al (111) collisions
at 2° incidence.
nance of the triplet core state over the singlet core
state [6,7,20,26,28]. The fact that the lowest 3s2
states and not other excited states are strongly
populated, has been explained by considering the
binding energies of the electrons in these states
with respect to the work function of the metal
surface [6 ]. In a static description, if the binding
energy is lower than the work function, resonant
ionization into the conduction band will occur and
hence predominantly the lowest-lying states should
be populated. More recently, other autoionizing
peaks have been observed [15,18,19,27] with a
typical intensity of almost two orders of magnitude
smaller than the main peaks. Three of these peaks
(b, c and f ) can also be observed in Fig. 1a. The
identification of the smaller peaks has been based
mainly on their energy position and is still a subject
of discussion [15,18,19,27].
In an attempt to obtain additional information
about the nature of these states and the dynamics
of their formation, in this work we performed a
study of the electron emission resulting from autoionization of neon scattered off an Al(111) surface
in a projectile energy range extending from 3 to
50 keV. The condition of glancing incidence allows
the reflection of most neon excited in collisions
with surface atoms and extends the interaction
time of the emerging projectile with the surface.
Even at high impact energies, the energy associated
with the motion of the projectile normal to the
surface can be reduced to a few eV by decreasing
the incident angle. Grazing collisions on smooth
surfaces also ensure that ions do not enter the
surface, making interpretation easier. The extended
energy range and the grazing condition allowed us
to determine the energy dependence of the production of the autoionizing peaks. By varying the
roughness of the surface, we investigated different
scattering conditions. Relatively rough surfaces
lead predominantly to hard collisions (single or
multiple) while smooth surfaces allow mainly soft
multiple collision sequences. For this latter case
the strong reflected beam has a narrow angular
distribution (typically 0.5–3°, depending on the
incident angle), which results in little Doppler
broadening of the peaks (although with a large
Doppler shift at our observation angles).
2. Experimental details
The experiments were conducted in an ultrahigh
vacuum chamber at an operating pressure of
3×10−10 Torr with the ion beam line open. The
ions were generated in a radio-frequency source,
mass-selected, and collimated to better than 0.1°.
The emitted electrons were analyzed with a
custom-made [36 ] cylindrical mirror energy spectrometer working at 1% energy resolution and
±1° angular resolution. The inner cylinder of the
spectrometer rotates around its main axis allowing
measurements over a wide range of observation
angles [36 ]. For the present measurements the
incident direction was selected to be random (i.e.,
not along a low-index crystallographic axis) and
the observation angle fixed at (h, w)=(21°, 5°),
O. Grizzi et al. / Surface Science 469 (2000) 71–79
with h measured from the ion beam direction and
w from the scattering plane (see inset of Fig. 1).
All of the electron energy spectra shown have been
corrected for the transmission function of the
spectrometer.
The Al(111) sample was prepared by repeated
cycles of grazing sputtering with 20 keV Ar+ ions
followed by annealing at 450°C. The azimuthal
orientation of the surface was continuously
changed during the argon-ion irradiation. This
method produces a surface topography that is
strongly dependent on the incident angle of the
argon-ion beam. At 0.5–2° incidence a very flat
surface can be obtained, while increasing the incidence angle to 3–4° may result in a considerably
rough surface. In this work we present measurements carried on both smooth and rough surfaces.
The characterization of the surface roughness was
performed by an atomic force microscope (AFM )
working in air, and in situ by measuring the
convoy electron peak produced by 60 keV protons
at 1° incidence [37]. This peak is due to electrons
captured to the continuum of the projectile, and
its energy position is critically dependent on surface roughness. The combined measurements of
AFM and convoy electrons [37] indicate that the
‘smooth’ surfaces discussed here consist mainly of
atomically flat terraces with average length of at
least 100 Å; i.e., larger than the interaction length
between a grazing ion and the surface. On the
other hand, the ‘rough’ surfaces have an average
roughness of 25 Å (root mean square of the height
distribution), corresponding to terraces of very
different size, which enhances the interaction of
the ion beam with steps or defects. The AFM
measurements show that, even for the rougher
surfaces used in the work, there are neither blisters
nor cones. The former are not formed because
most of the measurements and the cleaning cycles
are done at low incident angles, thus precluding a
large accumulation of argon or neon in subsurface
sites. The formation of cones is possible even at
low incident angles if the ion beam is left on a
fixed azimuthal direction for long periods. To
avoid this effect, we continuously rotated the azimuthal direction during the cleaning cycles. In all
cases, the surface cleaning was verified with Auger
73
electron spectroscopy before and after performing
the measurements.
3. Results
3.1. Projectile energy dependence of the neon
autoionization peaks
Typical electron energy spectra in the energy
range of autoionization of neon are shown in
Figs. 1a and b for a rough surface. The effect on
the electron spectra of smoothing out the surface
will be discussed in the next section. The spectrum
of Fig. 1b was taken with 20 keV Ne+ incident at
2° to the surface. The first interesting aspect is the
evolution of the autoionizing peaks with projectile
energy. The secondary peaks (a–h), hardly apparent below 5 keV, become prominent at higher
energies, as may be seen by comparing Figs. 1a
and b. We also note in the figures that these
secondary peaks are broader than the main peaks
(e.g., the width of line b is about 1.5 times that of
peak I or II ). Spectra taken at Ne+ energies
between 3 to 53 keV are shown in Fig. 2, after
background subtraction. The spectra are shifted
vertically by an amount proportional to the projectile energy, which aids the visualization of the
Doppler shift caused by the motion of the ions, as
is indicated for peak f by the parabolic dotted line.
To maintain the energy associated with the motion
of the projectile normal to the surface below 50 eV,
while keeping a high counting rate in the autoionizing peaks, we varied the incident angle from 6° at
3 keV to 1° at 53 keV.
The dependence of peak intensity and width on
projectile energy E shows the existence of at least
0
two groups of peaks. While the intensity of peaks
I and II reaches a maximum for E between 10
0
and 15 keV and then decreases, the intensity of
peaks a–h grows from a threshold at higher E
0
and remains high at the highest energies. This is
seen better in Fig. 3, which shows the peak intensity versus E (we do not present numerical data
0
for the low-intensity structures, d, e and g). As
mentioned above, the identification of the process
leading to peaks I and II is undisputed: autoionization of 2p4(3P)3s2(3P)2p5(2P) and 2p4(1D)3s2
74
O. Grizzi et al. / Surface Science 469 (2000) 71–79
Fig. 2. Electron energy spectra for Ne+–Al (111) collisions at
different energies (indicated by the labels) after subtraction of
a smooth background. The spectra were shifted vertically in
proportion to the projectile energy. The dashed line shows the
energy position of peak f shifted by the Doppler effect.
(1D)2p5(2P). In contrast, different interpretations have been offered for peaks a–h
[15,18,19,27]. In particular, Gallon and Nixon [15]
suggested that peaks b–e could be attributed to
2p4nln∞l∞ states, while Xu et al. [19] proposed that
peaks d–h could come from an initial 2p3 core
configuration. It was also suggested that peak b
could be due to decay of Ne11[2p4(3P)3s2(3P)] close
to the surface, from a numerical simulation analysis of shape of the autoionization peaks [24].
To determine the peak energy in the frame of
the excited neon, we corrected for the Doppler
shift. This procedure is simple with the glancing
angle geometry if we assume that projectiles scatter
specularly. This is a reasonable assumption at high
energies and for smooth surfaces. At low energies
or for rough surfaces we considered that peaks I
and II correspond to energies of 20.35 eV and
23.55 eV in the projectile frame. From the Doppler
shifts of these peaks we obtained the scattering
angles and then used these scattering angles to
calculate the energies in the projectile frame of the
Fig. 3. Intensity of the main autoionizing peaks (I, II ) and the
most intense secondary peaks (a–c, f and h) versus projectile
energy.
other autoionizing peaks. We note that the accuracy of the estimated peak energies (indicated in
Table 1) does not allow us to distinguish experimentally between different configurations having
similar energies. From an analysis of the peak
energy positions, there appear to be three possibilities of assigning parent states for the secondary
peaks, as given in Table 1. In some previous
reports, it was pointed out that the states
2s22p4nln∞l∞, possible parent states for peaks e–h,
could not account for the strong peak f since these
states have low ionization energies and therefore
would not survive resonant ionization to the solid.
However, this argument loses validity at high
projectile velocities, where one needs to consider
the effect of ion velocity: in the reference frame of
the projectile, the velocity of surface electrons is
shifted by the ion velocity. For electrons with
velocity V at the Fermi level (energy e ) there is
F
F
a smearing of the energy near the Fermi level of
De#2e (V /V ), or De (eV )#E1/2 , with E in keV,
F 0 F
0
0
for neon on aluminum. This smearing alters the
probabilities of resonant electron capture and loss
4.08
4.35
26.5±0.2 2p4(3P) 3p3p(3P) 3P, 3D 26.44
2p4(3P) 3p3p(3P) 1D
26.79
27.3
27.3
28.3
28.7
30.8
28.0±0.2 2p4 (1S) 3s22p4(1D)
3s3p(1P) 1F, 1P, 1D
2p4(3P)3p(4D)3p(3?)
2p4(3P)3p(2S)3p(?P)
30.2±0.2 2p4(1D) 3p3p 1S, 1D
31.9±0.2
35.2±0.2
d
e
f
g
h
3.21
6.96
3.21
2.58
2.68
4.86
4.56
25.72
5.99
6.96
25.5±0.1 2p4(1D) 3s3p(3P) 3F
2p4(1D) 3s3p(3P) 3P,3D
23.59
4.25
6.8
c
2p4(1D) 3s3s 1D
22.90
22.2±0.2 2p4(3P) 3s3p(3P) 3P,3D
b
II
20.39
20.3±0.1 2p4(3P) 3s3s 3P
I
30.6
30.7
31.8
31.7
32.3
32.5
32.0
35.0
35.1
35.8
35.2
35.2
35.6
2F
4D
2D
2D
2p3(2D) 3s3p 1P2p4 1D
2p3(2P) 3s23p2p4 (1D)3p
2p3(2D) 3s23p2p4 (3P)3p
2p3(2D) 3s23p2p4 (1D)3s
2p3(2D) 3s23p2p4 (3P)3p
2p3(2D) 3s3p (1P) 2p4 3P
2p3(2D) 3p2(iD) 2G 2p4 3P
2p3(2P) 3p2(3P) 4F 2p4 3P
2p3(2D) 3s23p2p4 (3P)3p 4P
2p3(2D) 3s23p2p4 (1D)3s 2D
2p3(2D) 3s23p 2p4 (3P)3s 4P
27.8
27.5
22.2
9.6
13.3
15.2
5.7
5.7
5.7
9.6
6.0
5.7
5.7
5.7
13.4
15.5
16.4
15.5
16.6
29.8
31.0
30.0
2s2p5(1P) 5p(2P) 2p4 (1D)
2s2p5(1P) 4p 2p4 (3P)
2s2p5(1P) 4s 2p4 (3P)
2s2p5(1P) 4s 2p4 (3P)
27.9
2s2p5(1P) 4p(2P) 2p4 (1D)
22.4
22.6
21.9
22.1
26.5
26.7
5p(4P) 2D2p4 (3P)
3p(2P) 2p4 (1D)
3s(2S ) 2p5 (2P)
3s(1S ) 2p5 (2P)
14.5
15.1
15.5
3.02
4.9
5.9
4.9
6.3
5.9
3.1
10.1
5.1
4.8
10.8
10.3
10.0
Predicted Ionisation
energy
energy
(eV )
(eV )
2s2p5(1P) 3d 2p4 (1D)
2s2p5(1P) 4s 2p4 (1D)
2s2p5(3P)
2s2p5(1P)
2s2p6(2S)
2s2p6(2S)
2s2p5(3P) 3p(4D) 2p4 (3P)
2s2p5(3P) 3p(4P) 2p4 (3P)
2s2p5(3P) 3p(2D) 2p4 (3P)
Predicted Ionisation Initial state
energy
energy
2s2p5, 2s2p6 cores
(eV )
(eV )
2p3(2D) 3s3p 4F2p4 3P
2p3(2P) 3s3s 2P2p4 3P
2p3(2D) 3s3s 2D2p4 3P
2p3(2P) 3s3s 2D2p4 1D
2p3(4S ) 3s3s 4S 2p4 3P
Predicted Ionisation Initial state
energy
energy
2s2 2p3 core
(eV )
(eV )
15.3±0.1 ---------
Initial state
2s2 2p4 core
2p5 (2P)
a
label Expt.
energy
(eV )
Table 1
Comparison of measured neon autoionization energies (after Doppler correction) and possible assignments obtained from Refs. [28–48]. The states included are those
with ionization energies higher than 2 eV
O. Grizzi et al. / Surface Science 469 (2000) 71–79
75
76
O. Grizzi et al. / Surface Science 469 (2000) 71–79
involving excited states of the projectile, depopulating states below the Fermi level and populating
states above the Fermi level. From the point of
view of partially blocking resonant ionization, De
corresponds to changing the work function of
aluminum from 4.3 eV to an ‘effective’ value of
3.3 eV at E =4 keV down to zero at E #18 keV.
0
0
This means that one cannot exclude the possible
population of low-binding-energy states at high
impact velocities. This consideration means that
the probability for the formation of states yielding
the secondary peaks should be somewhat dissimilar
from that of states giving the main peaks I and II
since, although the primary excitation mechanism
would be the same — i.e., involving the promotion
of the two electrons in the 4fs molecular orbital
(see below and [32–35] for details), the production
of the higher-lying states would become more
probable as the ion velocity increases. On the basis
of this discussion it appears appropriate to consider that the 2p4nln∞l∞ states given in Table 1 are
among the possible assignments for the secondary peaks.
One could, following Xu et al. [19], assign a
number of the secondary peaks to Ne+11 states
with a 2s22p3nln∞l∞ configurations. Thus peak b
(the strongest of the high-energy peaks) could
arise from the autoionization decay Ne+
[2p3(4S)3s2(4S)]Ne++[2p4(3P)]. The energy of
this transition has been observed at 22.45 eV in
4 keV Ne++–He gas-phase collisions [38]. In terms
of a molecular orbital description, used for this
type of collisions [32–35], the formation of excited
states with a 2s22p3 core is only possible for incident
ions (which carry a 2p vacancy). Indeed, in a neutral
neon collision, the initial quasi-molecular state has
a 3dp44fs2 configuration [19,27], while in the
Ne+ case there exist two possibilities corresponding
to the 3dp44fs1 and 3dp34fs2 states. Formation of
2s22p3 core states is only possible in the latter case;
it may be favored at high energies and grazing
angles, where scattering of the projectiles through
a sequence of two hard collisions is more probable,
allowing survival of the Ne+ formed in the first
collision. Peaks c and peak f could thus be assigned
[19] to Ne+[2p3(4S)3s3p] and Ne+[2p3(2P)3s2] or
[2p3(2D)3s3p], respectively.
Alternatively, one could assign most of the
peaks to states with a 2s vacancy: the
Ne+(2s2p5nl ) and Ne(2s2p6nl ) configurations
indicated in Table 1. In the molecular orbital picture, 2s vacancies are produced by rotational 3ps–
3pp coupling near the united atom limit, which
requires fairly small distances of approach. These
configurations were not expected in earlier studies
where the projectile energy used was too low for
2s excitation. On the other hand, evidence that 2s
electrons are in fact excited comes from an earlier
study [26 ] that detected vacuum ultraviolet radiation from the decay of, in particular, 2s2p6(3P)
Ne+1 and 2s2p5(3P) Ne++ states. For the present
measurements, considering the rather different
energy dependence of the intensities of different
excited states, we cannot rule out 2s excitation.
Therefore we propose that peaks a–h might have
a contribution from neon states with a 2s vacancy
even though, as stated before, we cannot rule out
contributions from 2s22p3 and 2p4 core states.
3.2. Dependence of peak intensities on surface
topography
The excitation of neon autoionization states
that decay outside the solid requires an incident
angle large enough to achieve the critical distance
of approach needed for excitation, but small
enough to ensure a high reflection coefficient.
These conditions are more easily achieved in collisions with rough surfaces at low incident angles,
where the projectiles can collide with the steps or
defects and then leave the surface faster than in
the case of flat surfaces. A discussion of this aspect
for ideally flat surfaces may be found in a study
combining trajectory simulations with a quasimolecular excitation model [27,39].
The effect of smoothing out or roughening the
surface in the spectra is shown in Fig. 4, for 20 keV
Ne+ at 2° incidence at different stages of the
surface preparation. Stage 1 corresponds to the
surface that is rough because it was prepared with
too few cycles of grazing ion sputtering and annealing. In stage 2 the number of cycles at 1.5°
incidence was increased, resulting in a smoother
surface. Stage 3 corresponds to a rougher situation
obtained by increasing to 4° the incident angle
used in sputtering, recovering approximately the
O. Grizzi et al. / Surface Science 469 (2000) 71–79
77
Fig. 4. Electron energy spectra for the rough surface at different
incident angles. Note that at 0.5° incidence there is already an
appreciable Auger peak from ejected Al 2p−1.
situation of stage 1. Stage 4 is the one that produces
the smoothest surface, with many sputtering–
annealing cycles (more than one week) at 1.5°.
Finally, stage 5 gives again the rough surface
situation after several cycles at 3–4° incidence. The
condition of stage 5 is the one used to obtain the
spectra shown in Figs. 1–3. We stress that, in all
of these cases, the spectra were obtained with
surfaces free of contamination (as seen with Auger
electron spectroscopy). The main effect of surface
condition observed in the spectra is that the peaks
decrease in intensity and broaden in smoother
surfaces.
Another evidence of surface roughness is the
presence of the Al 2p Auger peak at low incident
angles, arising from ejected Al(2p−1), since this
excitation requires rather close collisions (closer
than needed to excite autoionizing Ne11) that
cannot occur on smooth surfaces at low incidence
angles. This is illustrated in the electron spectra of
Fig. 5a, taken with 20 keV Ne+ at several incident
angles (corresponding to stage 5), which shows
the Al 2p Auger peak even at an angle of incidence
of 0.5°. One can also see that, while the intensity
of the Al 2p peak keeps increasing with incident
angle (up to 10–15°), the neon autoionization
peaks appear in a narrower region of incidence
angles, with a maximum around 2°. This maximum
Fig. 5. Electron energy spectra for rough (a) and smooth (b)
surfaces at different incident angles. Note that, for the rough
surface at 0.5° incidence, there is already an appreciable Auger
peak from ejected Al 2p−1, while for the smooth surface the
Al 2p Auger peak is very weak at 2° incidence.
shifts to higher incident angles for decreasing
projectile energy. In Fig. 5b, corresponding to
electron spectra measured as a function of the
incident angle for the smoothest case (stage 4), the
Auger peak from ejected Al 2p−1 is absent at 2°
incidence, indicating that violent collisions are
essentially suppressed for smooth surfaces.
The trends observed at high energies are reproduced at lower projectile energies. Fig. 6 shows
three spectra measured at 5 keV, the bottom one
corresponds to the roughest surface and the top
one to the smoothest. Similarly to the 20 keV case,
the autoionizing peaks broaden and become
weaker for smoother surfaces. The broadening of
the peaks for the smoother surfaces can be ascribed
to the fact that, in excitation collisions, scattering
occurs in a narrower angular range, which also
corresponds to smaller exit angles with respect to
78
O. Grizzi et al. / Surface Science 469 (2000) 71–79
Fig. 6. Electron energy spectra for 5 keV Ne+–Al (111) collisions at different stages of surface preparation (see text). The
upper spectra correspond to smoother surfaces.
the surface. As shown by some recent simulations
[24] of the characteristics of autoionization peak
shapes produced near surfaces, decay for exit
angles closer to the surface leads to broader peaks.
This is due to the broadening and shift of the
atomic levels produced by the atom–surface interaction, which leads to higher electron energies and
thus to a broadening of the spectral features. An
example of this effect comes from observations of
autoionization states produced close to the surface
during neutralization of Ne++ projectiles, which
yields broad peaks shifted to high energies [40,41].
In the case of a rough surface scattering occurs in
a broader angular range and a larger fraction of
excited atoms decay further from the surface leading to narrower peaks. The decrease in intensity
for flat surfaces is consistent with the larger distances of closest approach reached at grazing
incidence (particularly at low energies) and the
fact that the autoionizing states may be depopulated more easily by their close interaction with
the surface.
4. Conclusions
We have presented a study of the production of
neon autoionizing states on an Al(111) surface in
an extended projected energy range from 3 to
50 keV, on surfaces with different degrees of roughness. We find that the energy dependence of the
production of a series of low-intensity peaks is
quite different from that of the two main peaks
due to decay of 2p43s2 states, usually observed at
lower projectile energies. The secondary peaks
appear at higher energies and become very important at tens of keV.
Of the three possible parent core configurations
for the secondary peaks, the 2s22p4 core states can
be ruled out at the lower projectile energies because
their outer electrons would lie above the Fermi
level and would therefore not survive decay into
the solid by resonance ionization. On the other
hand, high projectile velocities allow a larger population of these states because of the kinematic
shifts of energy levels, an effect that would also
result in a different energy dependence for production of these states. The other two possibilities,
i.e., configurations with a 2s vacancy 2s2p5nl,
2s2p6nl and configurations 2s22p3nln∞l∞, could both,
in principle, account for many of the secondary
peaks. Nevertheless, existing data [26 ] on photon
emission in this energy range clearly show that
states with a 2s vacancy, 2s2p5(1P,3P)nl and
2s2p6nl are indeed excited at high energies.
A study of the effect of surface roughness shows
that a rougher surface favors production of the
autoionizing states in grazing scattering. It was
also found that, for flatter surfaces, the autoionization peaks become broader and less intense. We
ascribe this effect to excitation collisions closer to
the surface plane, which lead to autoionization
decay closer to the surface, where atom–surface
interactions broaden the states, consistent with
previous numerical simulations of the line shapes
[24,39,41].
Acknowledgements
This research was supported by the ECOSSECYT France–Argentine scientific collaboration
agreement, SECYT (PICTs 3-4-110 and 3-4220),
the Consejo Nacional de Investigaciones
Cientı́ficas y Técnicas of Argentina (CONICET )
423/98, and by a cooperative research grant from
O. Grizzi et al. / Surface Science 469 (2000) 71–79
the National Science Foundation and CONICET
(NSF INT-9724820, and PIP 0423/98).
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