Riccardi plasmons NI..

Nuclear Instruments and Methods in Physics Research B 164±165 (2000) 886±890
www.elsevier.nl/locate/nimb
Plasmon excitation in Al by keV Ne and Ar ions
P. Riccardi
a
a,*
, P. Barone a, M. Camarca a, A. Oliva a, R.A. Baragiola
b
Laboratorio IIS, Dipartimento di Fisica, Universit
a della Calabria, and INFM Unit
a di Cosenza 87036 Arcavacata di Rende, Cosenza,
Italy
b
Laboratory for Atomic and Surface Physics, University of Virginia, Engineering Physics, Charlottesville, VA 22901, USA
Abstract
We present experimental studies of plasmon excitation by keV Ne and Ar single charged ions on Al surfaces.
Plasmons are identi®ed through the characteristic energy spectrum of electrons from plasmon decay. At 1 keV incident
energy, surface plasmons are excited by Ne‡ but not by Ar‡ . For higher energies we observe bulk plasmon excitation
resulting, most likely, from excitation by fast electrons, such as Al-2p Auger electrons, traveling inside the solid. Ó 2000
Elsevier Science B.V. All rights reserved.
PACS: 79.20.Rf; 71.45.Gm; 73.20.Mf; 34.70.+e
1. Introduction
Plasmon excitation in ion-solid interactions is
attracting renewed interest since the experimental
discovery of the process of potential plasmon excitation [1]. For projectile velocities far below the
threshold for direct kinetic excitation [2], potential
plasmon excitation occurs at the expense of the
potential energy released upon neutralization of
the incoming ion by electron capture from the
solid. This mechanism thus competes with the
well-known Auger Neutralization (AN) process
[3±5]. AN can occur if En ˆ I 0 ÿ U > U, where En
is the maximum energy released in the neutralization event, U is the metal work function and I 0 is
*
Corresponding author.
E-mail address: riccardi@fermi.®s.unical.it (P. Riccardi).
the ionization potential of the parent atom shifted
by the image interaction D [3]. Ion neutralization
can be accompanied by plasmon excitation provided that En > Epl , where Epl is the plasmon energy. The plasmons then decay by excitation of
valence electrons [6,7] that may result in electron
emission, which is revealed by a characteristic
structure produced in the electron energy distribution [8]. This structure has a maximum energy,
Em ˆ Epl ÿ U, corresponding to the absorption of
the plasmon energy by an electron at the Fermi
level. Analysis of the energy distributions of electrons emitted by metal surfaces under slow noble
gas ions impact [1] reveals clear plasmon structures
for those materials that have sharp plasmon resonances, like the free electron metals Al and Mg.
In the case of Mg surfaces, the plasmon structure
induced by He‡ is well separated from the highenergy edge of AN, the energy separation being
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 1 1 2 7 - 1
P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 886±890
I 0 ÿ U ÿ Epl . Moreover, the structure due to electron emission from plasmon decay is more important than that of AN, thus leading to the
conclusion that potential plasmon excitation
should dominate the neutralization behavior
whenever energetically allowed.
Potential excitation of surface plasmons has
been the subject of several theoretical studies
[9,10]. Current theories do not predict bulk plasmon excitation by low energy ions that do not
penetrate inside the solid. However, the energy
distributions of electrons emitted by Al surfaces
under slow He‡ and Ne‡ impact [1] showed
structures whose energies were closely corresponding to that expected from bulk plasmon decay. Theoretical analysis by Monreal [11] showed
that neutralization may produce surface plasmons
of high momentum q, whose energies are shifted
approaching those of the q ˆ 0 bulk plasmon. However, experimental studies of surface
plasmon dispersion in Al have not shown such
high energies [12]. Theories have not considered
the e€ect of the hole produced in the solid during
neutralization.
Here we report energy distributions of electrons
emitted by Al surfaces under the impact of keV,
Ne and Ar single charged ions. It is shown that
potential excitation of surface plasmons occurs for
low energy Ne ions but not for Ar ions. At higher
energies, we observe bulk plasmon excitation most
likely resulting from fast electrons traveling
through the solid.
2. Experiments
The experiments were performed in an ultrahigh vacuum chamber with a base pressure of
3 10ÿ10 Torr Ne‡ and Ar‡ ions were produced in
a di€erentially pumped Atomika ion source by
electron impact. The discharge voltage was set at
35 eV for Ne‡ and 30 eV for Ar‡ in order to ensure
absence of double charged ions contamination in
the ion beam. The ion current was of the order of
10ÿ9 A and had a Gaussian spatial distribution in
both horizontal and vertical directions with a
FWHM less than 1 mm, as measured by a movable
Faraday cup at the target position.
887
A polycrystalline Al sample (purity 99.999%)
was mounted on a manipulator that allowed
variation of the ion incidence angle hi relative to
surface normal. The sample was sputter cleaned by
5 keV Ar‡ and Ne‡ ions, cleanliness was assured
by the absence of oxygen, and carbon signals in
Auger spectra induced by 2 keV electrons.
The emitted electrons were collected by a
hemispherical electrostatic energy analyzer
mounted on a goniometer. This analyzer, lying in
the incidence plane, has semi-acceptance angle of
1.5° and was operated at a constant pass energy
mode (DE ˆ 50 eV) and therefore an approximately constant transmission over the measured
energy range. Electron energies are referenced to
the vacuum level of the sample with an accuracy of
0.1 eV by comparing the energy of electrons from
autoionization lines of Ne 2p4 3s2 , with published
value [14]. To measure accurately low energy
electrons the chamber was shielded with l-metal to
reduce the e€ect of stray magnetic ®elds in the
neighborhood of the sample and analyzer.
3. Results and discussion
Fig. 1 shows representative electron energy
spectra of the electrons emitted from the Al surface by incident 1 and 5 keV Ne‡ ions and 2 keV
electrons. The electron induced spectrum shows
two structure commonly attributed to electron
emission from decay of q ˆ 0 surface and bulk
plasmon excited by fast electrons. The derivative
of the spectrum make these structures evident [8]
by revealing two minima at energies Em ˆ Epl ÿ U.
The minima occur at 6.5 and 11.2 eV for the surface and bulk plasmon, respectively.
Ion induced spectra compare well with previous
results [1,13] and show features due to kinetic
electron emission [19]: a low energy peak of cascade electrons and the two Ne** autoionization
lines around 20±25 eV [14]. The spectrum for
5 keV Ne‡ impact on Al shows a broad feature
whose derivative has a minimum closely corresponding in energy to that of the bulk plasmon in
the electron induced spectrum. This structure has
already been observed for 4.5 keV Ne‡ [13] and
attributed to bulk plasmon decay. A similar
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P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 886±890
Fig. 1. Top: energy spectra of electrons emitted from an Al
surface by the impact of 1 keV Ne‡ , 5 keV Ne‡ , and 2 keV
electrons using the geometry shown in the inset. Bottom: derivative dN(E)/dE that enhances the structure due to plasmon
decay.
structure appears in the spectrum induced by
1 keV Ne‡ . This structure was also attributed to
decay of bulk plasmons excited upon neutralization of the incoming ions [1]. However, in agreement with previous observations [1], we notice that
the minimum in the derivative spectrum for 1 keV
Ne‡ impact is shifted by about 1 eV to lower energy from that observed in the electron and 5 keV
Ne‡ spectra.
In previous research [15], we studied the variations with angle of incidence of the energy dis-
tributions of the electrons emitted from Al surfaces
under impact by 1±5 keV Ne‡ ions. We observed
that the energy and the intensity of the plasmon
structure for 1 keV Ne‡ impact is largely independent on the incidence angle. This ®nding suggested that plasmons are excited at or above the
surface and is consistent with the idea of excitation
by the shake up due to the sudden disappearance
of the dipole formed by the incoming ion and the
image charge [1]. The observation that the energy
of the plasmon excited by 1 keV Ne‡ ions is lower
than that of even the q ˆ 0 bulk plasmon, along
with the observation that excitations occur at or
above the surface, supports the interpretation of
surface plasmons of high momenta, postulated by
Monreal [11]. This would imply that surface plasmons can exist with energies higher than previously reported.
At higher impact energies a transition from
surface plasmon excitation to bulk plasmon excitation occurs. Angular measurements for 5 keV
Ne‡ on Al have shown that the plasmon energy is
close to that of the q ˆ 0 bulk plasmon and that
plasmon intensities increase as the incidence angle
deviates from the surface normal. This is consistent with the idea that plasmon excitation occurs
mainly in the bulk of the solid and that the intensity of the emission from plasmon decay depends on the depth were excitation occurred.
In Fig. 2, we report energy distributions of
electrons emitted from Al surfaces by Ar‡ ions as a
function of incident energy. Ions were impinging
on the surface at an incidence angle hi ˆ 60° and
the observation angle was he ˆ 0°. The spectra are
shown normalized so that their integrals equal
known electron yields [16]. The main feature of the
spectrum induced by 1 keV Ar‡ on Al is the broad
distribution that results in the minimum at about
5 eV in the derivative spectrum. We have shown
[15] that Ar‡ neutralization at Al surface is not
mediated by plasmon excitation and the minimum
at 5 eV in the derivative correspond to the high
energy edge of Auger neutralization. As the ion
energy increases, the spectra show the evolution
from the potential electron emission to the kinetic
electron emission regime. The dip in the derivative
at 5 eV decreases and a broad structure appears. This structure results in the minimum in the
P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 886±890
889
Fig. 3. Plasmon yields cp versus incident energy for Ar‡ impact
on Al. cp has been evaluated following the procedure outlined
in [16].
Fig. 2. Top: energy spectra of electrons emitted from the Al
surface by keV Ar‡ impact. The parameter is the ion energy in
keV. The spectra have been normalized so that their integrals
equal known electron emission yields. Bottom: derivative
dN(E)/dE.
derivative observed around 11 eV. Comparison
with Fig. 1 shows that the energy of this structure
corresponds to that of the structure appearing in
the spectrum of the electrons emitted by an Al
surface under electron and 5 keV Ne‡ irradiation.
Therefore, it is assigned to electron emission from
bulk plasmon decay. Bulk plasmon excitation in
experiments of Ar‡ ions bombardment on Al have
been observed in the past [17] but at much higher
incident energies than those reported here. A dip
in the derivatives close to 6.5 eV due to a q ˆ 0
surface plasmons is apparent in the spectra, but
cannot be clearly resolved.
The results of Fig. 2 indicate that bulk plasmon
excitation in the case of Ar ions on Al is related to
kinetic electron excitation. This idea is also supported by the behavior of the intensity of the plas-
mon structure as a function of ion energy, plotted in
Fig. 3. Plasmon intensities have been evaluated
following the procedure outlined in [18]. The energy
dependence suggests that there is a threshold incident energy for bulk plasmon excitation. A precise
evaluation of this threshold is made dicult because electron emission from bulk plasmon decay is
masked by the background of secondary electrons.
To understand the origin of this kinetic plasmon
excitation we note that so-called plasmon loss satellites have been observed in the past in the Al-2p
Auger spectra excited by electron impact as well as
by electron promotion in violent atom±atom collisions in our energy range [20,21]. Our data are
consistent with the idea that, at the threshold,
plasmon may be excited by fast Auger electrons
traveling inside the solid, consistent with the explanation for sub-threshold plasmon excitation by
keV protons recently published [22].
In conclusion, we have studied the excitation of
Al plasmons by keV Ne‡ and Ar‡ ions. In the case
of low energy Ne‡ on Al, data are consistent with
theoretical prediction of potential excitation of
surface plasmons of short wavelength. At higher
incident energies, bulk plasmon excitation occurs
and the reported data show that plasmons can be
eciently excited by fast electrons traveling inside
the solid.
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P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 164±165 (2000) 886±890
Acknowledgements
This research has been supported by Istituto
Nazionale per la Fisica della Materia, INFM and
the National Science Foundation.
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