Nuclear Instruments and Methods in Physics Research B 182 (2001) 89±95
www.elsevier.com/locate/nimb
Mechanisms for plasmon production by hollow atoms
above and below an Al surface
N. Stolterfoht
a,*
, J.H. Bremer a, V. Ho€mann a, M. R
osler a, R. Baragiola b,
I. De Gortari a,c
a
b
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
c
Instituto de Fisica, Laboratorio de Cuernavaca, 62191 Cuernavaca, Mexico
Abstract
Low-energy electrons ejected by 1±4 keV Ne4‡ ion impact on an Al surface were measured to study spectral
structures near 11 eV attributed to the decay of plasmons. Absolute electron yields from the plasmon decay were
primarily studied as a function of the incidence angle of the Ne4‡ projectile. Model calculations were performed to
separate contributions from ions penetrating into the solid and ions re¯ected at the surface. Opposite energy dependencies were obtained for the two contributions suggesting that they are governed by potential and kinetic energy
e€ects, respectively. Ó 2001 Published by Elsevier Science B.V.
1. Introduction
Valence electrons in metals can take part in
quantized collective oscillations known as plasmons whose formation by charged particle impact
can be described within the framework of the freeelectron gas approximation [1,2]. The decay of
plasmons occurs predominantly by energy transfer
into a single valence electron in an interband
transition [3]. 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.
Early experiments with ion impact have been
*
Corresponding author.
E-mail address: [email protected] (N. Stolterfoht).
performed using fast projectiles that create plasmons via direct Coulomb excitation [4]. Only recently, an increasing attention has been devoted to
plasmon production by slow projectiles.
For ions with energies below a few keV, various
groups have considered the process of plasmonassisted capture where the transfer of potential
energy from the projectile produces a plasmon
[5±12]. Particular attention has been focused on
slow heavy ions incident on a surface with a highcharge state [7,13±15]. The characteristic feature of
a highly charged ion is its large potential energy
which has prompted much interest in the ®eld of
ion±solid interactions [16]. Above the surface,
highly charged ions strongly attract several electrons, which are resonantly captured into highRydberg states whereas inner shells remain empty.
Thus, the projectiles evolve into hollow atoms
0168-583X/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V.
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N. Stolterfoht et al. / Nucl. Instr. and Meth. in Phys. Res. B 182 (2001) 89±95
whose formation and decay imply various novel
processes.
The mechanisms for plasmon production resulting in spectral structures at low electron energies are still under debate. In particular,
controversial ideas have been raised in view of
structures observed near 11 eV for Al surfaces.
Apart from Auger capture processes [17], the decay of both bulk plasmons [6,7,14] and surface
plasmons [8,10,11] have been considered to be responsible for the structure near 11 eV. The production of bulk and surface plasmons was found
to be dependent on the incident energy and angle
[6]. The interpretation of the 11 eV structure as a
surface plasmon [8] appeared to be con¯icting,
since it is usually found at smaller energies near 6.5
eV [4].
In the present work, to gain more information
about highly charged ions, we study structures in
the electron spectra near 11 eV by impact of 1±4
keV Ne4‡ on Al as a function of the incidence
angle of the projectile. First results of our measurements have been given before [18]. For the
analysis of the data, two regions relevant for
plasmon production are considered. As shown in
Fig. 1 these regions are located above the surface
near the jellium edge and below the surface, i.e.
below the ®rst atomic layer. These regions are
likely to be associated with the production of
surface and bulk plasmons, respectively. It is
shown that the di€erent types of plasmon production can be separated by means of model calculations.
2. Experimental method and results
The measurements were carried out at the 14.5
GHz electron cyclotron resonance (ECR) source at
the Ionenstrahl-Labor (ISL) in Berlin using an
ultra-high vacuum chamber equipped with a rotatable electron spectrometer [19]. Details of the
experimental method have been presented before
[13,14,20]. A beam of Ne4‡ ions was collimated to
a diameter of about 1 mm and directed onto a
clean Al(1 1 1) target. The pressure in the chamber
was a few 10 10 mbar. Electrons emitted from the
target were measured using an electrostatic parallel-plate spectrometer. The spectrometer eciency
and the ion current were determined so that absolute values for electron emission yield could be
measured [20]. The experimental set-up was optimized to reliably measure these electron yields at
energies as low as 2±4 eV. Examples for results of
absolute electron yields N …e; X† ˆ dY =de dX for
incidence angle w ˆ 3°, 7° and 20° are given in
Fig. 2.
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 that Al are produced by L-Auger transitions ®lling 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 Ne in the initial state
Fig. 1. Diagram to visualize the two regions for plasmon production considered in the theoretical analysis. The projectiles are re¯ected
at the surface with the probability PR and, hence, they enter into the solid with the probability 1 PR . Re¯ected Ne atoms survive the
passage through the above-surface region in a doubly excited state with the probability PD .
N. Stolterfoht et al. / Nucl. Instr. and Meth. in Phys. Res. B 182 (2001) 89±95
91
background from other processes, it is a common
practice to di€erentiate the measured electron
intensities N …e; X†. Results for the derivative
dN =de are given in Fig. 3 for 4 keV Ne4‡ impact
on Al. The graphs labeled (a), (b) and (c) on the
left-hand side show dN =de for incidence angles of
w ˆ 3°, 7° and 20°, respectively, relative to the
surface plane. The observation angle of the electrons is equal to b ˆ 70° relative to the surface
normal. For the de®nitions of the angles see also
the inset in Fig. 2.
Each dN =de curve clearly shows a structure
near 11 eV which is attributed to plasmon decay
[7,14]. As discussed in detail previously [14], the
plasmon structure appears as a negative peak (dip)
when performing the derivative of the electron
intensity. This dip was separated from the electron
background by ®tting the intensities above and
Fig. 2. Electron yields measured for 4 keV Ne4‡ impact on Al as
a function of the electron energy. The incidence angles are
w ˆ 3°, 7° and 20° as indicated. The peak labeled Al is produced by L-Auger transitions ®lling 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. Note that the spectra for 3° and 7° are multiplied
by 4 and 2, respectively.
1s2 2s2 2p4 3s2 1 D decaying into 1s2 2s2 2p5 2 P under
emission of a 22 eV Auger electron [21]. 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 from about 5 to 14 eV [2,4]. In the following we shall focus on the plasmon structures
near 11 eV, since the spectral features around 6.5
eV were found to be weak and not reproducible.
To enhance the visibility of the plasmon structures, which are superimposed on an intense
Fig. 3. Derivative of electron yields measured for 4 keV Ne4‡
impact on Al as a function of the electron energy. In (a), (b) and
(c), data are given for the incidence angles w ˆ 3°, 7°, and 20°,
respectively, as shown in the left-hand side graphs. On the righthand 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 182 (2001) 89±95
below the plasmon structure by a second-order
polynomial (given in Figs. 3(a)±(c) as dashed
lines). The uncertainties of the plasmon intensity
due to the background subtraction are typically
25%. On the right-hand side, the corresponding
plasmon dip structures are shown as obtained after
subtraction of the background intensity. It is seen
that the centroid energy of the dip is slightly
shifted as the incidence angle increases. This shift
is larger than the uncertainties in the dip positions
possibly caused by the background subtraction.
Furthermore, it is found that the dip intensity for
the grazing angle of w ˆ 3° is signi®cantly smaller
than those for w ˆ 7° and 20°.
For further analysis of the data we integrated
the peak structures after background subtraction.
As shown in detail previously [14], the electron
yields from plasmon decay can be obtained by
multiplying the integrated results by the quantity 6.3 eV. This value is equal to 2EF =3
…EF being the Fermi energy† reduced by 15%
due to electron attenuation e€ects. In Figs. 4(a)±(c)
absolute values of the electron yield dY =dX are
plotted for projectile energies of 1, 2 and 4 keV,
respectively. The dY =dX curves show strong variations with the incidence angle w. After a region of
nearly constant values, the yield decreases to a
minimum and rises again at small angles, whereby
the location of the minimum changes with the
projectile energy. To interpret the present observations we performed model calculations described in the following.
3. Interpretation and discussion
As in previous works [5±7] 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 should be considered. Note ®rst that Ne
orbitals higher than the 2p shell cannot participate
in the bulk plasmon creation, since those orbitals
are not bound inside the solid [22]. However, energetic electrons produced directly in collisions as
Fig. 4. Plasmon yield from Al for Ne4‡ impact as a function of
the incidence angle w. In (a), (b) and (c), data are given for
impact energies of 1, 2 and 4 keV, respectively. The curves are
due to model calculations indicating the contributions from
above and below the surface (see text).
well as Auger electrons may excite plasmons when
traveling through the solid [14,23].
For a better understanding of the data, we
consider two regions relevant for plasmon production as shown in Fig. 1: above the surface near
the jellium edge and below the surface de®ned by
the ®rst atomic layer. These contributions are
likely to be associated with surface and bulk
plasmons, respectively. One may argue that the
consideration of surface plasmons is inconsistent
with the present observations. In Fig. 3(a) the 11
eV structure observed for w ˆ 3° is signi®cantly
larger in energy than Al surface plasmons found at
6.4 eV [4]. However, as emphasized previously
[8,10], the latter energy refers to plasmons of near
zero momentum, whereas the decay of surface
N. Stolterfoht et al. / Nucl. Instr. and Meth. in Phys. Res. B 182 (2001) 89±95
plasmons with larger momentum may produce
electrons of higher energies. Moreover, high-momentum transfer may result in the production of
surface plasmons of higher multipoles [10]. The
occurrence of electrons from high-momentum
surface plasmons would be consistent with the
shift of the plasmon dip observed in Fig. 3. At
small incident angles (w ˆ 3°) the electron spectrum is composed of both surface and bulk plasmons whereas at higher incident angles (w ˆ 7°
and 20°) the bulk plasmons dominate. Apart from
plasmon decay, one may consider that similar to
He [17] the Auger capture into Ne may give rise
to electrons with energies near 11 eV for Al.
However, in the case of Auger transitions involving a double folding over the Fermi edge we expect
structures broader than those produced by plasmon decay. Hence, in the following, we shall not
consider Auger capture contributions to the 11 eV
dip.
In the following, the electron yields due to
above- and below-surface processes are denoted
dYa =dX and dYb =dX, respectively. These yields are
governed by the re¯ection probability PR of the
projectile at the surface, which strongly depends
on the incidence angle w. In a simple model we set
dYb
ˆ B…1
dX
PR †;
…1†
where B is a constant assuming that the ions
penetrating into the solid produce bulk plasmons.
Similarly, we set
dYa
ˆ A PR …1
dX
PD †;
…2†
where A is a constant and PD is the survival
probability for the hollow neon atom moving
through the jellium region of thickness d that is
relevant for plasmon assisted capture (i.e. emerging into the vacuum with at least one vacancy in
the L-shell). By de®nition these ions are lost for
plasmon production.
In the analysis, the re¯ection and survival
probabilities are determined from other sources,
whereas A and B are treated as free parameters.
We assume an exponential decay law
PD ˆ exp… u=vz †;
…3†
93
where u is a constant velocity and vz ˆ v sin w is the
vertical velocity of the projectile. It can readily be
shown that u 2dCL is obtained as the double
passage of the jellium edge region within the lifetime of the hollow Ne atom. Both the velocity u
and the re¯ection probability PR were determined
from an independent analysis of the atomic Ne
Auger peak observed at 22 eV in Fig. 2. As pointed
out already, this peak can be used to obtain information about the degree of projectile scattering
at the surface. This empirical method of determining the ion scattering is advantageous, as it
takes into account the speci®c properties of the
surface (such as the surface roughness).
Finally, the sum dYa =dX + dYb =dX was ®tted to
the experimental data treating A and B as adjustable parameters for each incident energy. The results are plotted in Fig. 4 where apart from the
sum (solid lines) also the individual yields dYa =dX
(dashed lines) and dYb =dX (dash±dotted lines) are
given. It is seen that the theoretical data compare
well with experimental providing con®dence for
the essential features of the present model. The
comparison explains detailed features of the experimental results. For instance, the existence of a
minimum in the electron yields and its shift with
the projectile energy can be associated with the
onset of specular re¯ection of the projectiles at the
surface.
The main goal of the present analysis is the
(approximate) separation of above- and belowsurface contributions to the plasmon yield. We
note that these contributions are governed by the
parameters A and B, respectively, which are
obtained as asymptotic electron yields A ˆ dYa
…w ! 0†=dX and B ˆ dYb …w 0†=dX. Fig. 5
shows these parameters as a function of the projectile energy. It is found that the yields A and B
have opposite energy dependencies, i.e. A decreases slightly, whereas B increases signi®cantly
with increasing projectile energy. The decrease of
A with energy con®rms potential energy e€ects
being responsible for the above-surface processes.
On the other hand, the increase of B with energy
suggests that the below-surfaces processes are in¯uenced by kinetic energy e€ects. This ®nding is
consistent with our recent analysis of absolute
plasmon yields for di€erent incident charge states
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N. Stolterfoht et al. / Nucl. Instr. and Meth. in Phys. Res. B 182 (2001) 89±95
impact of multi-charged ions. Absolute values for
the experimental electron yield from plasmon decay have been deduced using an approximate
model. An analytic expression was derived to estimate the contribution of the above- and belowsurface processes on the plasmon production.
However, we feel that the interpretation of the 11
eV structure as a surface plasmon, which usually
occurs at 6.4 eV, remains to be debated.
The comparison between theory and experiment suggests that the contributions to plasmon
production from below and above surface change
signi®cantly when the angle of incidence reaches
that of specular re¯ection. The most striking
®ndings are the di€erent energy dependencies of
the above- and below-surface contributions con®rming potential energy mechanisms, however,
pointing also to other mechanisms for plasmon
production. In view of the present results we feel
that future work is motivated to study the opposite
energy dependencies of the contributions to plasmon production above and below the surface.
Fig. 5. Electron yields dY =dX due to the decay of plasmons
obtained from the results given in Fig. 4. In (a) and (b) the data
refer to the asymptotic values of A ˆ dYa =dX at w ! 0 and
B ˆ dYb =dX at w ! 25° likely to be associated with surface and
bulk plasmons, respectively.
[14] suggesting that for charge states higher than
two the plasmons are produced indirectly by secondary electrons. Indeed the Al L-Auger electrons
near 63 eV (Fig. 2) and the continuum electrons
above that energy increase in intensity as the
projectile velocity increases.
4. Concluding remarks
Summarizing, the present study is concerned
with experimental and theoretical e€orts to clarify
mechanisms for plasmon creation by multiply
charged neon moving slowly in Al. Primary attention is devoted to capture processes which
provide the potential energy necessary for the
plasmon creation. We achieved progress in the
understanding of plasmon production by slow
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