Surface Science 480 (2001) L420±L426
www.elsevier.nl/locate/susc
Surface Science Letters
The excitation of collective electronic modes in Al by
slow single charged Ne ions
P. Barone a, R.A. Baragiola b, A. Bonanno a, M. Camarca a, A. Oliva a,
P. Riccardi a,b,*, F. Xu a
a
Laboratorio IIS, Dipartimento di Fisica, Universita 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, Thornton Hall B-103,
22901 Charlottesville, VA, USA
Received 15 November 2000; accepted for publication 1 March 2001
Abstract
We have studied electron emission from decay of plasmons excited in the interactions of slow single charged Ne ions
with Al surfaces as a function of incident energy ranging from 0.7 to 8 keV. Bulk plasmon excitation occurs above a
threshold incident energy of 1 keV and initially coexists with potential excitation of surface modes. The excitation of
bulk plasmons in Al is determined by fast electrons traveling inside the solid. Ó 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Ion±solid interactions; Secondary electron emission; Plasmons; Aluminum; Noble gases; Polycrystalline surfaces
Electron emission is a fundamental process in
slow ion±solid interactions. The phenomenon is
generally attributed to two main excitation mechanisms [1]. In potential electron emission, electron excitation results when the potential energy
brought by the incoming ion is released upon
neutralization by electron capture from the surface. In kinetic electron emission, electron excitation results from the transfer of kinetic energy
from the ion. Our basic understanding of both
*
Corresponding author. Address: Material Science and
Engineering, University of Virginia, Thornton Hall B-103,
Charlottesville, VA 22903, USA. Tel.: +1-804-924-1059; fax:
+1-804-924-1353.
E-mail addresses: [email protected], riccardi@®s.unical.it
(P. Riccardi).
kinetic and potential electron excitation mechanisms is currently undergoing a substantial development, after the recent observations of surface
and bulk plasmon excitation in the interaction of
slow ions with surfaces of free electron metals [2±
4]. The detection of plasmon excitation relies on
the fact that the main plasmon decay channel is the
excitation of valence electrons (interband transitions) [5], that can result in electron emission with
a characteristic energy distribution. The maximum
energy of this structure, Em ˆ Epl / [5], where
Epl is the plasmon energy and / is the metal work
function, corresponds to the case where the plasmon energy is absorbed by an electron at the
Fermi level.
Energy and momentum conservation require
a minimum threshold velocity vth for plasmon
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
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excitation due to the motion of charges in metals.
For ions (much more massive than electrons), the
threshold velocity for this ``kinetic'' plasmon excitation is vth 1:3 vF [6], where vF is the Fermi
velocity. Direct kinetic excitation is thus unimportant for projectile velocities lower than vth and
the observed plasmon excitation needs to be attributed to other mechanisms.
For slow projectile ions that do not penetrate
inside the bulk, plasmon excitation results from
the conversion of the potential energy En released
upon neutralization of the incoming ions [2]. Potential plasmon excitation occur if En Epl . It thus
competes with the well known Auger neutralization process [7] and dominates the neutralization
behavior whenever energetically allowed [2]. Theoretical calculations [8] suggest that this mechanism may excite monopole surface plasmons of
high momentum q whose energy is thus shifted
approaching that of the q ˆ 0 bulk plasmon. Results of angular studies of electron emission [3]
showed that potential plasmon excitation occurs
indeed at or above the surface and the excited
plasmons can be either the predicted monopole or
the multipole surface plasmons [9]. So far, this
latter collective excitation, recently observed in
electron energy loss experiments on Al surfaces
[10], has not been taken into account in calculations.
Bulk plasmon excitation has been recently reported in experiments of ion bombardment on Al
and Mg surfaces in the keV energy range. Several
excitation mechanisms have been considered to
discuss the experimental results, such as neutralization inside the solid when multiply charged Ne
ions interact with Al surfaces [4], excitation by fast
electrons excited in the electronic collision cascade
induced by projectile ions [11,12] and excitation of
high momentum bulk plasmons accompanied with
momentum transfer to the target atoms for keV
proton impact on Al(1 1 1) at grazing incidence
[13].
In a previous study [3] of Ne‡ impact on Al
surfaces, we found that a transition from surface
to bulk plasmon excitation occurs as the energy of
the ions is increased from 1 to 5 keV. A detailed
study of this transition is needed to understand the
possible mechanisms of these sub-threshold bulk
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plasmon excitations in ion±solid interactions. To
this end we performed measurements of energy
distributions of electrons emitted by Al surfaces
under the impact of keV Ne‡ ions as a function of
incident energy. Bulk plasmons were found to be
excited above a threshold incident energy that can
be estimated to be about 1 keV. The increase of
electron emission from bulk plasmon decay with
incident energy indicates that plasmon excitation is
related to kinetic electron excitation. The estimated threshold energy is close to the threshold
energy for Al-2p Auger electron excitation by
electron promotion [14] in violent atom±atom collisions, thus suggesting that fast Auger electrons
are responsible for sub-threshold bulk plasmon
excitation by singly charged noble gas ions.
The experimental setup has been described
elsewhere [3,12]. Brie¯y, beams of Neon ions were
produced in a di€erentially pumped Atomika ion
source whose discharge voltage was set to 35 V to
prevent the formation of double charged ions. The
ion beams were collimated to a diameter of less
than 1 mm and directed onto a sputter cleaned
polycrystalline Al target. The emitted electrons
were collected by an electrostatic energy analyzer
with a semi-acceptance angle of 1:5° and operated
at a constant pass energy mode (DE ˆ 50 eV) and
therefore at an approximately constant transmission over the measured energy range. The UHV
chamber was shielded with l-metal to reduce the
e€ect of stray magnetic ®elds on electron trajectories.
In Fig. 1 we report energy distributions of
electrons emitted from Al surfaces by Ne‡ ions as
a function of incident energy. Ions impinged on
the surface at an incidence angle Hi ˆ 60° and the
observation angle was He ˆ 0° (angles are measured with respect to surface normal). The spectra
have been normalized so that their integrals equal
known electron yields [15,16]. The reported spectra
show characteristic features of kinetic electron
emission: a low energy peak due to electrons produced in the electronic collision cascade inside the
solid and the two discrete lines around 20±25 eV
due to autoionizing decay of Ne excited by electron promotion in violent atom±atom collisions
[17]. Consistently with previous experiments [2±4],
the spectra show a prominent shoulder at 9±12 eV
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Fig. 1. Energy spectra N …E† of electrons emitted from the Al Surface by keV Ne‡ impact. The spectra have been normalized so that
their integrals equal known electron emission yields [15,16].
attributed to electron emission from plasmon
decay. The visibility of this structure is usually
enhanced by taking the derivative of the spectra.
The shoulder in the spectra results in the minima
in the derivative at energies Em ˆ Epl / [5]
shown in Fig. 2.
The spectrum acquired at the lowest incident
energy of 0.7 keV used in these experiments shows
a shoulder which results in the minimum at about
10.5 eV in the derivative, i.e. about 1 eV less than
the energy of the q ˆ 0 Al bulk plasmon (15.5 eV
minus / ˆ 4:3 eV for Al). This energy value suggests that the structure is more likely due to electron emission from decay of multipole surface
plasmons excited at or above the surface [3] by
potential energy transfer upon neutralization of
the incoming ions. At the highest incident energy
of 8 keV the observed plasmon structure appears
at an energy closely corresponding to that of the
q ˆ 0 Al bulk plasmon, consistently with previous
observations [2,4]. The spectra acquired at intermediate energies show the transition from surface
to bulk plasmon excitation. As the energy of the
incident ions increases, we notice from Fig. 2 that
the structure due to bulk plasmon decay grows on
the high energy side of the surface plasmon decay
structure. A similar behavior was already appearing in the data reported in Ref. [2] but not analyzed as such.
Information about intensity of electron emission
from plasmon decay can be obtained by analyzing
the structure produced in the derivative of the
spectra [18]. In panels (a)±(d) of Fig. 3 are reported
examples of polynomial background subtraction
from the derivative of the spectra. The uncertainty
in the background subtraction is estimated to be
30% by varying the function representing the
background and the regions on both side of the
plasmon structure to which the ®tting procedure is
applied. The corresponding negative peaks after
subtraction are shown in panel (e)±(h) of Fig. 3. It
is evident from panel (e) of Fig. 3 that at 1 keV
incident energy the structure is well reproduced
by one gaussian curve centered at an energy below that corresponding to a bulk plasmon decay structure and assigned to decay of multipole
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Fig. 2. Derivative dN …E†=dE of the spectra in Fig. 1 that evidentiate the transition from surface to bulk plasmon excitation. The dotted
vertical lines are used to guide the eye and the derivatives are arbitrarily displaced in the vertical scale for clarity.
surface plasmon. At incident energies J 5 keV
(panel (h)) the structure is reproduced by a
gaussian at the position of the bulk plasmon. The
position and width of this gaussian remain nearly
constant with increasing incident energy. At intermediate incident energies, as shown in panel (f)
and (g) of Fig. 3, the structure is not reproduced
by only one curve but by the superposition of two
gaussians of constant width and position corresponding respectively to the surface and bulk
plasmon features.
Absolute yields for surface and bulk plasmon
decay electrons can be calculated from the areas
ASP , ABP of the corresponding gaussians by multiplying by a factor C which is independent on the
excitation mechanisms since the plasmon is an
intrinsic property of the target. The value C ˆ
2EF =3 ˆ 8 eV, where EF is the Fermi energy, has
been recently estimated [18] by a simple model
which includes electron attenuation e€ects but not
scattering and cascade of electrons in the solid.
Electron cascade is taken approximately into account in the theory by Chung and Everhart [5],
which produces derivative spectra in agreement
with experiments for electron impact. From their
results, we obtain a value C 40 eV. Given this
uncertainty on the value of C, we just plot, in the
upper panel of Fig. 4, the areas ASP and ABP of the
gaussians corresponding respectively to the surface
and bulk plasmon decay features.
We observe that the growth of the bulk plasmon
structure occurs for incident energies above a
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Fig. 3. (a±d) Examples of polynomial background subtraction from the derivative dN …E†=dE; (e±h) Gaussian curve ®ts of the negative
peaks obtained after background subtraction.
threshold incident energy of about 1 keV and
dominates the spectrum for incident energies
above 2 keV. The comparable intensity of the two
plasmon structures at incident energies right above
the threshold is an important ®nding. At these
energies, in fact, neutralization inside the solid is
not expected to be an ecient mechanism for bulk
plasmon production. This is because the majority
of the Ne ions are neutralized to the ground state
in the incoming trajectory before undergoing hard
collisions with target atoms as shown by several
experiments [19±26].
On the other hand, bulk plasmons can be eciently excited by electrons traveling inside the
solid with energies greater than a threshold value
of about 35 eV [11]. This mechanism was indeed
observed to contribute to bulk plasmon excitation
in the case of multiply charged ion bombardment
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atom collisions, if the internuclear distance of the
two colliding partner is smaller than a critical
value Rth, i.e. if the incident energy is above a
threshold value Eth. Threshold energies from
Auger intensity measurements have been well
studied in the past and the values reported in literature [28,29] are consistent with the estimated
threshold for bulk plasmon excitation reported
here.
In conclusion our experiments point to a simple
physical picture for plasmon excitation which is
consistent with previous experiments of slow single
charged Ne ions interacting with Al surfaces. At
low incident energies, potential excitation of surface plasmons occurs above the surface if the potential energy En released upon neutralization of
the incoming ions exceeds the plasmon energy.
Penetration inside the bulk of incoming particles
results in an electronic collision cascade in which
bulk plasmons are excited by excited electrons.
The threshold behavior of bulk plasmon excitation
can be determined by Al LVV Auger electrons
excited by electron promotion in collisions between two target atoms.
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Fig. 4. Top: Areas of the surface (ASP ) and bulk plasmon (ABP )
gaussians described in Fig. 3 versus Ne‡ incident energy. The
total electron emission yield (ctot ) [15,16] and the yield (cLMM )
[28,29] of Al-Auger electron emission are also reported after
proper rescaling. The lines through data points are used to
guide the eye; bottom: ratio R ˆ ABP =ctot versus incident Ne‡
incident energy.
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