Nuclear Instruments and Methods in Physics Research B 193 (2002) 523–529
www.elsevier.com/locate/nimb
Auger transitions and plasmon decay produced by
hollow atoms at an Al(1 1 1) surface
N. Stolterfoht
b
a,*
€sler a, R.A. Baragiola
, J.H. Bremer a, V. Hoffmann a, M. Ro
b
a
Hahn-Meitner-Institut Berlin GmbH, Bereich Strukturforschung, Glienickerstr. 100, D-14109 Berlin, Germany
Laboratory for Atomic and Surface Physics, University of Virginia, Engineering Physics, Charlottesville, VA 22901, USA
Abstract
Low-energy electron emission induced by 1–4 keV Ne4þ ion impact on an Al surface was analyzed. Distinct lines
observed near 22 eV are attributed to Auger electrons ejected from doubly excited Ne2 decaying well above the surface.
Spectral structures near 11 eV are primarily associated with the decay of bulk plasmons. After instrumental effort of
extending the capability of the spectrometer to measure electrons with energies as low as 2 eV, we made observations of
intense spectral structures near 6.5 eV, which are likely to be due to the decay of surface plasmons. Absolute electron
yields were studied as a function of the incidence angle of the Ne4þ projectile and were interpreted by means of model
calculations. Ó 2002 Elsevier Science B.V. All rights reserved.
PACS: 79.20.Rf; 29.20.Ap; 34.50.Dy
1. Introduction
During the last decade considerable effort has
been devoted to studies of the interaction of slow
highly charged ions with surfaces (see [1,2] and
references therein). These studies have revealed
that multiply charged ions strongly attract several
electrons, which are resonantly captured into high
Rydberg states whereas inner shells remain empty.
Thus, the projectiles evolve into hollow atoms
whose formation and decay imply various novel
processes. From the experimental point of view,
electron spectroscopy has been proved as a powerful tool to obtain information about the processes occurring during the approach of a highly
*
Corresponding author.
E-mail address: [email protected] (N. Stolterfoht).
charged ion at a surface [3–8]. In particular, Auger
electron spectroscopy has been used by several
groups to reveal the dynamics of formation and
decay of hollow atoms at the surface and in the
bulk of the solid [1,2].
Recently, an increasing attention has been devoted to plasmon production in metals by slow
projectiles. Plasmons are quantized collective oscillations of valence electrons whose excitation by
charged particle impact can be described within
the framework of the free-electron gas approximation [9–11]. The decay of plasmons occurs
predominantly by energy transfer into a single valence electron in an interband transition [12].
Hence, electrons of characteristic energies are
ejected from the metal providing a signature for
plasmons, which can experimentally be studied by
means of electron spectroscopy [13]. For ions with
energies below a few keV, various groups have
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 8 3 1 - 5
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N. Stolterfoht et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 523–529
considered the process of plasmon-assisted capture
where the transfer of potential energy from the
projectile produces a plasmon [14–22]. In particular, the attention has been focused on slow heavy
ions incident on a surface with a high charge state
[16,23–26], since such ions have a large potential
energy.
The mechanisms for plasmon production in Al
resulting in spectral structures at 11 eV are still
under debate. The decay of both bulk plasmons
[15,16,24] and surface plasmons [17,19,20] have
been considered to be responsible for this phenomenon. However, the interpretation of the 11 eV
structure as a monopole surface plasmon is problematic, since its energy is expected in the vicinity
of 6.5 eV [13]. Therefore, it was postulated that the
structures near 11 eV are due to a composition of
bulk plasmons and multipole surface plasmons
[20,21]. Similar effects have been considered for
impact of highly charged ions [26]. At 6.5 eV certain structures have been found in previous work
[16,27], however, no detailed studies have been
devoted to surface plasmons in this energy range.
In the present work, with an improved spectrometer capability to measure low-energy electrons, we studied structures in the spectra using
1–4 keV Ne4þ impact on Al. First, to gain information about the amount of ions scattered from
the surface, the distinct peak at 22 eV due to Auger
electron emission from doubly excited neon is
analyzed. Then, electron intensities due to plasmon decay are studied in the low-energy range of
the electron spectra. Apart from the well-known
bulk plasmon structure near 11 eV we observed
intense structures in the vicinity of 6.5 eV where
surface plasmons are expected.
2. Experimental method and results
The measurements were carried out at the
14.5 GHz electron cyclotron resonance source at
the Ionenstrahl–Labor in Berlin using an ultrahigh vacuum (UHV) chamber equipped with a
rotatable electron spectrometer. Details of the experimental method have been presented before
[23,24,28]. A beam of Ne4þ ions was collimated to
a diameter of about 1.5 mm and directed onto a
clean Al(1 1 1) target. The pressure in the chamber was a few 1010 mbar. Electrons emitted from
the target were measured using an electrostatic
parallel-plate spectrometer. The spectrometer efficiency and the ion current were determined so that
absolute values for electron emission yield could
be measured [28].
The experimental set-up was optimized to reliably measure electron yields at low energies. It
consists of a UHV chamber with an efficient Mumetal shielding to reduce the magnetic field. The
wall of the UHV chamber is built of Mu metal of
2 mm thickness and the horizontal openings, lid
and bottom, are closed by Mu metal of the same
thickness [28]. Close to the target this Mu-metal
shielding allows for the reduction of the earth
magnetic field to a few mG. In addition, we removed insulators in the vicinity of the target to
avoid spurious electric fields by electrical charge
up. For the present experiments we carefully
screened all cables carrying voltage. Hence, we
expect that our system is capable to measure reliable electron intensities at energies as low as 2 eV.
Examples for results of absolute electron yields
N ðe; XÞ ¼ dY =de dX for incidence angles w ¼ 10°,
20°, 30°, and 45° are given in Fig. 1. (For the
definitions of the angles see also the inset in the
figure.) The electron spectra exhibit various
structures, which can be attributed to emission of
Auger electrons as well as the decay of plasmons
produced within the surface and the bulk. The
peaks denoted Al are produced by L-Auger transitions filling a vacancy in the L-shell of Al. The
peaks labeled Ne2 , Ne3 , Ne4 are due to L-Auger
transitions in hollow Ne with 2, 3 and 4 vacancies
in the L-shell, respectively. The distinct Ne2 peak
can be attributed to above-surface emission of
Auger electrons from doubly excited neon scattered from the surface [29]. The appearance of this
peak shows that a noticeable fraction of the projectiles captures two electrons into the L-shell and
undergo an L-Auger transition well above the
surface. The intensity of the Ne2 peak was analyzed to obtain information about the amount of
projectiles scattered at the surface, as described
further below.
Structures due to the decay of surface and bulk
plasmons can be observed in the low-energy range
N. Stolterfoht et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 523–529
Fig. 1. Electron yields measured for 4 keV Ne4þ impact on Al
as a function of the electron energy. The incidence angles are
w ¼ 10°, 20°, 30° and 45° as indicated. The peak labeled Al is
produced by L-Auger transitions filling a vacancy in the L-shell
of Al. The peaks labeled Ne2 , Ne3 , Ne4 are due to L-Auger
transitions in hollow Ne with, respectively, 2, 3 and 4 vacancies
in the L-shell.
from about 4 to 14 eV [10,13]. Indeed, one observes a hump near 11–12 eV, which can be attributed to bulk plasmons possibly containing a
component of multipole surface plasmons [26]. In
addition, near 6.5 eV a further hump is visible
which can well be identified on top of the background of secondary electrons. These electrons are
likely to be associated with surface plasmons. It
should be noted that in our previous work [16,24]
we did not observe the structure at 6.5 eV as
clearly as in the present spectra. Our present experience shows that the 6.5 eV structure could only
be clearly identified after the experimental set-up
was improved with respect to the measurement
of low-energy electrons.
3. Auger electron emission
As noted in Fig. 1 the maxima labeled Neq with
q ¼ 2, 3 and 4 are due to Auger electron emission
525
from hollow neon with q vacancies in the L-shell.
The label Neq shall indicate that the neon is
neutral, i.e. the electrons missing in the L-shell are
transferred into the M or C shell, depending on
whether the decay occurs above or below the
surface, respectively [31,32]. The structures attributed to Ne3 and Ne4 are found to be relatively
broad, indicating that these projectiles are likely to
decay below the surface. In contrast, the decay of
Ne2 atoms give rise to a rather distinct line at
22 eV, which can be associated with the initial state
1s2 2s2 2p4 3s2 1 D decaying into 1s2 2s2 2p5 2 P under
emission of Auger electron [29]. The occurrence of
an atomic Auger transition provides evidence that
Ne2 atoms decay well above the surface where its
influence has diminished. This picture is supported
by a Doppler analysis of the 22 eV electrons, which
revealed that the decaying projectile moves away
from the surface, i.e. it is scattered from the surface. A further analysis showed that the distinct
Ne2 peak is superimposed on a broader structure,
indicating that part of the Ne2 atoms decay also
below the surface.
The distinct line at 22 eV allows for the determination of the fraction of Ne2 atoms scattered
from the surface. After background subtraction
and integration of this line, absolute Auger electron yields were obtained as shown in Fig. 2. The
data are plotted as a function of the vertical energy
Ez of the incident Ne4þ ions (Ez ¼ Ep sin2 w, where
w is again the incidence angle). It is found that Ez is
a suitable scaling parameter for the present electron yield. In fact, Fig. 2 shows that the Auger
electron yields for different projectile energies from
1 to 4 keV follow one universal curve. This curve
drops at low and high values of Ez forming a
maximum in the intermediate energy region.
To understand this energy dependence of the
Auger electron yields, we consider two regions of
the surface relevant for different trajectories of the
projectiles. As shown in Fig. 3 these regions are
located above the surface near the jellium edge
and below the first atomic layer. The incoming
ions are assumed to be reflected at the first atomic
layer with the probability pR and, hence, the
ions enter into the bulk with the probability 1 pR
(Fig. 3). When reflected, the ions traverses the
jellium edge whereby the doubly excited state of
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N. Stolterfoht et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 523–529
ability pD . Then, the electron yield dYNe2 =dX due
to above-surface Auger emission from Ne2 can be
written as
0
dYNe2 dYNe
2
¼
pR pD ;
dX
dX
Fig. 2. Differential yields dYNe2 =dX for Auger electron emission from doubly excited Ne2 as a function of the vertical
energy Ez ¼ Ep sin2 w. The projectile energy Ep is 1–4 keV. The
incidence angle w is shown for 4 keV at the top scale of the
figure. The dashed-dotted is a fit through the experimental data.
The curve labeled Ne2 survival represents the yield pD =4p of
Ne2 Auger electrons for ions reflected at the surface (see also
text). Te curve labeled ‘‘Reflected’’ referring to the right-hand
scale represents the fraction pR of reflected ions.
Fig. 3. Diagram to visualize the two regions for plasmon production considered in the theoretical analysis. The projectiles
are reflected at the surface with the probability pR and, hence,
they enter into the solid with the probability 1 – pR . Reflected
atoms Ne2 survive the passage through the above surface region in a doubly excited state with the probability pD .
Ne2 is quenched, e.g. by electron capture or resonant ionization. Finally, a remaining fraction of
Ne2 atoms emerge from the surface with the prob-
ð1Þ
0
where dYNe
2 =dX stands for the normalized angular
distribution of the 22 eV Auger electrons. As0
suming isotropic emission it follows that dYNe
2 =
dX ¼ 1=4p. Expression Eq. (1) already explains
the basic feature of the Ne2 decay curve in Fig. 2.
The probabilities pR and pD are decreasing and
increasing functions with Ez , respectively, and
hence their product exhibits a maximum in an intermediate region.
For a more quantitative analysis, we used the
cascade model [32] introduced to describe the
decay of hollow atoms in front of the surface and
inside the bulk. The results are given in Fig. 2 as
the dashed line labeled ‘‘Ne2 survival’’. Since we
have no room for a detailed description of the
cascade model calculations, we provide a few explanations to make this curve plausible. After Ne2
is produced close to the surface, its quenching in
the jellium edge may be described by an exponential decay function expðu=vz Þ where u is a
constant velocity and vz ¼ vp sin w is the vertical
velocity of the projectile. Using u as an adjustable
parameter, it is readily verified that the Ne2 survival curve follows an exponential law. In particular, it confirms that the vertical velocity (or
energy) is a suitable scaling parameter (Fig. 2).
After determination of the Ne2 survival function, the reflection probability pR was deduced
from the present data. The experimental data were
fitted by a smooth function (dashed dotted line in
Fig. 2) which, in turn, was divided by the Ne2
survival function. The results are given as the solid
line labeled ‘‘Reflection’’ referring to the righthand scale of Fig. 2. The solid line represents the
fraction of ions scattered at the first atomic layer.
As expected this curve is close to unity at small
scattering angles and it decreases monotonically
with increasing scattering angle. We note that the
present empirical method of determining the ion
reflection is advantageous, as it takes into account
the specific properties of the surface such as the
surface roughness.
N. Stolterfoht et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 523–529
4. Surface and bulk plasmons
Structures due to the decay of surface and bulk
plasmons can be observed in the low-energy range
from about 5 to 14 eV [10,13]. To enhance the
visibility of the plasmon structures, which are su-
527
perimposed on an intense background from other
processes, it is common practice to differentiate the
measured electron intensities N ðe; XÞ ¼ dY =de dX.
In Fig. 4 results are given for the derivative dN =de
obtained using 4 keV Ne4þ impact on Al. On the
left-hand side, the graphs labeled (a), (b), (c) and
Fig. 4. Derivative of electron yields measured for 4 keV Ne4þ impact on Al as shown in Fig. 1. In (a), (b), (c) and (d) data are given for
the incidence angles w ¼ 10°, 20°, 30° and 45°, respectively, as shown in the left-hand side graphs. On the right-hand side the plasmon
structures are shown after subtraction of the background.
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N. Stolterfoht et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 523–529
(d) show dN =de for incidence angles of w ¼ 10°,
20°, 30° and 45°, respectively, relative to the surface plane. The observation angle of the electrons
is equal to a ¼ 30° relative to the surface plane.
(For the definitions of the angles see also the inset
in Fig. 1.)
The derived plasmon structure appears as a
negative peak (dip) whose analysis was discussed
in detail previously [24,27]. This dip was separated
from the electron background by fitting the intensities above and below the plasmon structure by
a second-order polynomial (given in Fig. 4(a–d) as
dashed lines). The uncertainties of the bulk plasmon intensity due to the background subtraction
are typically 25%. Due to the uncertainties of
measuring low-energy electrons, the surface plasmons may be affected by systematic errors which
are difficult to estimate. On the right-hand side of
Fig. 4 the corresponding plasmon dip structures
are shown after subtraction of the background
intensity. It is seen that the centroid energy of the
dip is slightly shifted as the incidence angle increases [26]. While this shift lies within the experimental uncertainties, it is noted that similar
phenomena of plasmon dip shifts have been observed elsewhere [21,26].
For a better understanding of the data, we reconsider the two types of ion trajectories shown in
Fig. 3. The surface plasmons are primarily produced by ions reflected at the surface while passing
through the jellium edge. However, surface plasmons are also created by ions entering into the
bulk when they are still above the surface. This
may explain the nearly constant surface plasmon
yield with increasing incidence angle. The bulk
plasmons are expected to be produced uniquely by
ions penetrating into the surface. At large incidence angles the ions penetrate deeply into the
bulk so that the electrons ejected by the plasmon
decay are increasingly absorbed in the bulk. At
small incidence angle the ions are increasingly reflected from the surface so that the production of
bulk plasmons is suppressed. Thus, it is plausible
that the bulk plasmon yield maximizes at intermediate angles (Fig. 4).
As in previous work [14–16] we consider excitation by potential-energy transfer as a unique
mechanism for plasmon production by slow heavy
ions. This mechanism involves the capture of a
valence electron into the L-shell of the Ne projectile, which provides the energy for plasmon
creation. However, other mechanisms for plasmon
creation must be considered. Note that Ne orbitals
higher than the 2p shell cannot participate in the
bulk plasmon creation, since those orbitals are not
bound inside the solid [30]. On the other hand,
energetic electrons produced directly in collisions
as well as Auger electrons may excite plasmons
when traveling through the solid [24,27,33].
In [26] we inferred from the experimental data
measured at small incident angles that the plasmons are produced by potential-energy effects
whereas at larger angles the bulk plasmons are
created by secondary electrons. This latter finding
is consistent with our previous analysis of absolute
plasmon yields for different incident charge states
[24] suggesting that for charge states higher than 2
the plasmons are produced indirectly by secondary
electrons. In the latter case it is expected that also
surface plasmons are produced. The fact that the
surface plasmons observed here at 6.5 eV are relatively strong suggests that for heavy ions impact
additional mechanisms are producing these plasmons. It remains to verify whether the monopole
surface plasmons are also created by potentialenergy effects.
5. Concluding remarks
Summarizing, the present study is concerned
with experimental effort to analyze Auger electron
and plasmon creation by multiply charged neon
moving slowly at an Al(1 1 1) surface. Earlier
studies [16] of multiply charged neon ions incident
on Al with a few keV have focused on the structure
at 11 eV which is usually attributed to bulk plasmons. In those studies only weak indications for
monopole surface plasmons were observed. There
is no principal reason for a suppression of surface
plasmons. For instance, when plasmons are produced by secondary electrons, the occurrence of
surface plasmons is expected with about the same
probability as bulk plasmons [11]. In this work
we have indeed observed pronounced structures,
which may be attributed to surface plasmons. This
N. Stolterfoht et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 523–529
observation is believed to be possible after instrumental efforts of removing disturbing fields in the
target region. However, since the measurements
of low-energy electrons are always affected by
uncertainties, further work is needed to confirm
the present observations.
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