Nuclear Instruments and Methods in Physics Research B 157 (1999) 110±115
www.elsevier.nl/locate/nimb
Mechanisms for ion-induced plasmon excitation in metals
R.A. Baragiola
a,*
, S.M. Ritzau a, R.C. Monreal b, C.A. Dukes a, P. Riccardi
a,c
a
Laboratory for Atomic and Surface Physics, University of Virginia, Engineering Physics, Charlottesville, VA 22901, USA
Dept. Fõsica Te
orica de la Materia Condensada, Universidad Aut
onoma de Madrid, C±V, Cantoblanco, 28049 Madrid, Spain
Dipartimento di Fisica, Universit
a degli Studi della Calabria, and INFM, Unit
a di Cosenza 87036, Arcavacata di Rende (CS), Italy
b
c
Abstract
We have studied the excitation of plasmons produced by 100 eV He‡ , Ne‡ and Ar‡ and by 5±100 keV H‡ and He‡
projectiles in Al and Mg through the observation of electrons from plasmon decay, ejected from clean and cesiated
surfaces. At low velocities, plasmon excitation occurs only for ions of high potential energy and is independent of
velocity. The e€ect of Cs adsorption on this potential plasmon-excitation mechanism on Al surfaces suggests that the
excited plasmons are not bulk plasmons, as was assumed previously, but short-wavelength surface plasmons. For ions
moving faster than a threshold velocity vth 1.3 vFermi predicted by electron gas theories, kinetic plasmon excitation can
occur because the valence electrons cannot respond instantaneously to screen the moving charge. We found that,
contrary to theoretical expectations, plasmon excitation by H‡ and He‡ projectiles occurs below vth . With the aid of a
simple model, we suggest that this sub-threshold excitation results from energetic secondary electrons. Ó 1999 Elsevier
Science B.V. All rights reserved.
PACS: 71.45.Gm; 73.20.Mf; 79.20.Nc; 34.70.+eF
1. Introduction
It was recently discovered that neutralization
accompanied by plasmon excitation is an important electron transfer process at surfaces of freeelectron metals, for ions carrying high potential
energy [1,2]. This process, termed potential plasmon excitation, can occur at velocities lower than
the threshold expected for kinetic plasmon excitation. In the kinetic mechanism, a major energy
*
Corresponding author. Tel.: +1-804-982-2907; fax: +1-804924-1353; e-mail: [email protected]
loss process for fast charges penetrating condensed matter [3±5], plasmons can be excited because the valence electrons cannot respond
instantaneously to screen the moving charge. The
threshold velocity, vth , is determined theoretically
from conservation of energy and momentum assuming direct Coulomb interactions of the fast
charge with an electron gas. For heavy ions
bombarding solids with low plasmon damping, vth
1.3 vFermi .
Several recent theoretical papers have been
published on potential excitation of surface plasmons during surface neutralization [6±10]. This
process competes with more studied neutralization
processes, like Auger neutralization (AN) or res-
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 4 3 0 - 9
R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 110±115
onance neutralization followed by Auger de-excitation [11]. This latter process is very important in
low work function surfaces, like those resulting
from alkali adsorption. AN can occur if the
maximum energy released upon neutralization of
the incoming ion, En ˆ I0 ÿ /, is larger than /,the
work function of the surface. Here I0 is the neutralization energy of the ion I minus the image
interaction (2 eV). Plasmons can be excited
provided En > Epl , the plasmon energy. With a
work function of 4.3 eV for Al, slow He‡ (I ˆ 24.6
eV) and Ne‡ (I ˆ 21.6 eV) can excite the bulk
plasmon of Al but Ar‡ (I ˆ 15.8 eV) cannot, in
agreement with observations [1] (E0plb ˆ 15.3 eV for
k ˆ 0 and increases with momentum transfer k)
[12]. On the other hand, excitation of surface
plasmons in Al, and surface and bulk plasmons in
Mg is allowed for the three ions (E0pls ˆ 10.6 eV)
[1].
Plasmon excitations have been studied mainly
theoretically since their detection in experiments is
indirect, relying on the observation of the ejected
electrons resulting from plasmon decay [3,4]. The
characteristic energy distribution of electrons from
this decay makes it possible to separate them from
electrons originating from other processes such as
AN, ionizations in ion±atom and ion±electron
collisions, and Auger decay of inner-shell excitations [11]. Plasmon-assisted neutralization is also
distinguishable from AN because it occurs later,
after the plasmon lifetime [13]. We note that other,
indirect, evidence of plasmon excitations appear in
the energy loss, scattering and electron emission in
the interaction of fast molecular ions with solids
[14±17].
Kinetic excitation of plasmons has been described theoretically in recent papers [18±21] and
observed in a few experiments on fast ion impact
on metals at energies of tens and hundreds of keV
[22±25] which have been limited to identifying the
plasmon decay structures. Here we report new
experimental results of studies designed to test the
dependence of plasmon excitation on the state of
the surface and the velocity of the ion. We analyze conditions for potential excitation of plasmons and provide evidence for kinetic plasmon
excitation below the theoretically predicted
threshold.
111
2. Results and discussion
The measurements of the energy distributions
of electrons ejected by ion impact were performed
in two UHV systems. For energies <5 keV, the
setup and methods have been described previously
[1] with the addition of our ability to deposit cesium on the sample with sub-monolayer control.
For energies >5 keV we use the second chamber,
which is attached to a 100 kV ion accelerator. Ions
are incident at 60° to the sample normal and
electrons are detected in the direction perpendicular to the surface with a hemispherical energy
analyzer. Samples in this second chamber are
produced by in situ vapor deposition and the
cleanliness is monitored by Auger electron spectroscopy.
In our previous studies of potential plasmon
excitation we have shown that for slow He‡ ions
on Mg, the electron structure due to plasmon decay is more important than that of AN [1]; Fig. 1
shows that the di€erent groups of electrons are
well-separated in energy. It is important to note
that electron energies from AN depend on the
potential energy of the ion whereas plasmon decay
energies are intrinsic to the sample, i.e., independent of the type of ion, assuming the same momentum is transferred. The high-energy edge is
indicative of Auger neutralization, which causes a
dip in the derivative dN/dE at I0 ÿ 2/ [11] or 11
eV for Ne‡ and 14 eV for He‡ , separated by the
di€erence of the ionization potential. This highenergy edge is broadened by the incomplete
adiabaticity caused by the ®nite ion velocity normal to the surface [26]. In addition to the Auger
neutralization edge, a prominent shoulder is observed. Its position, 7 eV, is not correlated with I;
hence it is not due to AN involving structure in the
density of valence states and is assigned to plasmon excitation. The high-energy edge of the
shoulder is at Em ˆ Epl ÿ /, which corresponds to
the case where the plasmon is absorbed by an
electron at the Fermi level. The energy separation
to the high-energy cuto€ of AN is I0 ÿ / ÿ Epl and
this is the reason why the two structures are more
clearly separated in Mg than in Al. The plasmon
edge is broadened by a constant value, given by
the ®nite lifetime of the plasmon, and does not
112
R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 110±115
Fig. 1. Electron energy spectra N(E) for 106 eV ions and 1 keV electrons on Mg and Be, together with their derivatives dN(E)/dE
(bottom). The vertical scales in the Be spectra are in arbitrary units. Plasmon structure is seen in the Mg (shoulder at 7 eV) but not in
the Be spectra.
increase with the velocity of the ion, like the
broadening of the AN edge.
Fig. 1 shows the case of Be, where only Auger
neutralization is seen (the structure in the derivative shifts with the potential of the ion), in contrast
to Mg, where one can see plasmon structure that is
®xed in energy. The absence of a clear plasmon
structure in Be is likely associated with the much
larger width of the plasmon compared with that of
Mg and Al [5], which lowers the value of the derivative. The visibility of plasmon decay thus becomes clearer in those materials that have a sharp
plasmon resonance. Solids that should have clear
plasmon decay structure are those that have sharp
plasmon resonances. They include, besides Al and
Mg, other alkaline earths, Si and alkalis. The absence of a clear structure does not mean that
plasmons are not excited. In fact, we expect that
plasmon excitation should dominate the neutralization behavior in all cases allowed by energy
conservation. In addition, it is possible that plasmon excitation also accompanies Auger de-excitation of excited atoms at surfaces.
In our previous paper [1], we noted that plasmon energies were close to but slightly lower than
those of long-wavelength bulk plasmons, which
are the ones more easily excited by fast charges.
The excitation of bulk plasmons is unexpected,
since neutralization occurs most likely when the
ion is outside the surface. Current theories of
R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 110±115
plasmon excitations by external charges do not
predict bulk plasmon excitation outside the solid.
A possibility is that what we attribute to a bulk
plasmon is in reality a surface plasmon of short
wavelength, as suggested by Monreal [9]. To resolve this question we have produced slight alterations of the surface by sub-monolayer adsorption
of Cs that, as veri®ed by low energy electron energy loss spectroscopy, a€ects the surface but not
the bulk plasmon. In these experiments the Cs
coverage was monitored by the change in work
function, D/ (the maximum change, D/ ˆ 2.94 eV,
occurs at a coverage of about half a monolayer).
The sample was biased negative to allow the collection of all electrons. Fig. 2 shows that the
plasmon structure disappears after a very small Cs
coverage. This sensitivity to a change in the surface
electronic structure suggests that the plasmons
reported in our previous work [1] were surface
plasmons, shifted by energy dispersion that occurs
Fig. 2. Electron emission from an Al surface bombarded with
106 eV He‡ ions for di€erent Cs coverages characterized by a
decrease in the work function, D/. The sample was biased by
ÿ5 V, and the vertical scales have been displaced for clarity.
Notice the quick disappearance of the plasmon structure near
15 eV upon Cs adsorption. The structure near 10.5 eV is related
to Cs.
113
at large momentum transfers, in accordance with
recent calculations [9].
Previously, we found that potential plasmon
excitation was independent of ion velocity, up to
5 ´ 107 cm/s [1]. Our new experiments for He‡ on
Al extend to 2 ´ 108 cm/s vFermi . The energy
distributions for 20 keV He‡ (Fig. 3) show that
bulk plasmon decay structures are excited below
the predicted threshold velocity for kinetic excitation. This is also the case for H‡ , where potential excitation cannot occur at v ˆ 0 and should
be weak even considering kinematic shifts of energy levels [27]. Fig. 4 shows the ratio of the intensity of the plasmon structure to the integral of
the total energy distribution (the electron yield).
The plasmon decay intensity dcp is computed for
Fig. 3. Electron energy spectrum (top) and derivative (bottom)
for Al excited by H‡ and He‡ at 20 keV, which travel with a
velocity less than vFermi .
114
R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 110±115
Fig. 4. Ratio of plasmon decay yields to total electron yields as
a function of projectile velocity, for H‡ and He‡ on Al. The
plasmon yield dcp is obtained in a window 2 eV wide, centered
at the plasmon decay edge. The dashed lines are the calculated
contributions of excitations by secondary electrons. The solid
lines are to guide the eye through the experimental data, and
have no other meaning.
a ‹ 1 eV energy ÔwindowÕ around the plasmon
edge after subtracting the tail of high-energy
electrons. In this way we avoid uncertainties
caused by uncertainties in the shape of the plasmon structure.
There are several possible mechanisms to explain this unexpected result of excitation below vth .
First, one needs to consider the ®nite plasmon
width that results from plasmon damping. However, this can only introduce a small correction to
vth , since, for these metals, the ratio of plasmon
width to plasmon energy is very small. Then, one
can consider that the constraints of energy and
momentum conservation can be relaxed if one allows a target atom to absorb some of the momentum. However, this is not expected to be
important, as judged from the similarity in the
calculated energy loss of slow protons in an electron gas and in a crystal, which includes lattice
e€ects [28]. Another factor to consider is that the
velocity distribution of valence electrons is displaced in the frame of the moving projectile. This
can then allow the potential mechanism to occur,
since it e€ectively increases the potential energy
available in the neutralization process by
'mvvFermi , where m is the electron mass. An additional e€ect that can contribute is the energy
uncertainty caused by the ®nite ion velocity normal to the surface [29]. But electron capture accompanied by plasmon excitation is not limited to
the surface; it can also occur inside solids, as has
already been described theoretically [30].
Finally, plasmons can be excited by fast secondary electrons excited by the projectile. These
fast electrons can result from several collision
processes. Direct ionization may occur by binary
ion-electron collisions or by electron promotion in
the case of multi-electron projectiles. Also, fast
electrons will result from Auger processes following the excitation of an inner-shell of the projectile
or the target. To test excitation by fast electrons
one can look for a correlation between the number
of ejected electrons with sucient energy to excite
a plasmon and the number of plasmon decays in a
given energy spectrum of electrons. A better test
requires modeling the energy distribution of fast
electrons inside the solid from the observed distribution of ejected electrons. This can be done
using electron-cascade models based on mean free
paths for inelastic electron scattering. A fast electron produced at a depth z inside the metal will
reach the surface without degrading its energy with
probability exp(ÿz/L)/2 and will lose energy by
exciting a plasmon at a depth z0 in a di€erential dz0
with probability exp[ÿ(z ÿ z0 )/L]dz0 /lp , with L and
lp being the total inelastic mean free path and the
plasmon excitation mean free path, respectively.
By considering all possible values of z and z0 we
can relate the number of excited plasmons to the
number of energetic electrons ejected from the
solid. The results are shown in Fig. 4. Details of
the calculation were recently published [27]. It is
seen that excitation by fast secondary electrons can
account for plasmon excitation at velocities lower
than the threshold for direct excitation.
R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 110±115
In conclusion, our new experiments have determined that a sharp plasmon resonance is necessary for the observation of plasmon decay
structure in the energy spectra of secondary electrons. We expect that plasmon excitation should
be important in neutralization whenever it is energetically allowed. This implies that most theories
of Auger neutralization in ion-surface collisions
need to be re-examined. Finally, we have found
that kinetic plasmon excitation can occur for
projectile velocities lower than the theoretical
threshold, due to the e€ect of energetic secondary
electrons excited directly by the projectile.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Acknowledgements
This work was supported by NSF-DMR,
SWRI, Iberdrola S.A., and the Spanish Comisi
on
Interministerial de Ciencia y Tecnologõa, contract
PB97-0044.
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