Nuclear Instruments and Methods in Physics Research B 212 (2003) 339–345
www.elsevier.com/locate/nimb
Bulk and surface plasmon excitation in the interaction
of Heþ with Mg surfaces
P. Riccardi
a,b,*
,
A. Sindona a, P. Barone a, A. Bonanno a,
A. Oliva a, R.A. Baragiola b
a
b
Laboratory for Atomic and Surface Physics, University of Virginia, Engineering Physics,
22901 Charlottesville, Virginia, USA
Laboratorio IIS, Dipartimento di Fisica, Universita della Calabria and INFM – Unita di Cosenza,
87036 Arcavacata di Rende, Cosenza, Italy
Abstract
We report on studies of plasmon excitation in Mg under the impact of slow Heþ ions in the incident energy range
0.1–0.5 keV. Consistent with studies on Al surfaces, a transition from surface to bulk plasmon excitation occurs as the
energy of the ion is increased. Previous methods in the literature to obtain information about yields of electron emission
from plasmon decay are discussed. A new data analysis procedure is presented, which allows disentangling the contributions to the experimental spectra arising from different emission mechanisms, showing that potential excitation of
multipole surface plasmons gives an important contribution to electron emission.
Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction
Neutralization by electron capture of slow ions
at metal surfaces releases potential energy that can
excite plasmons [1], quantized collective oscillations of the metal charge density. Plasmons of
energy Epl can be excited if En Epl , where En is
the potential energy transferred to the solid. The
excited plasmons then decay by excitation of valence electrons resulting in electron emission,
which is experimentally identified by a characteristic structure produced in the energy distribution
of emitted electrons. This structure shows a highenergy edge at Em ¼ Epl U [1,2], where U is the
*
Corresponding author.
E-mail address: riccardi@fis.unical.it (P. Riccardi).
metal work function, corresponding to the emission of an electron from the Fermi level.
Plasmon excitation and decay competes with
the well-known mechanisms for potential electron
emission at metal surfaces [3,4]: Auger neutralization, that involves two electrons from the solid,
and resonant neutralization followed by interatomic Auger deexcitation. Since the work by
Baragiola and Dukes [5], suggesting that electron
emission from plasmon decay is more important
than Auger neutralization, several investigation
[6–12] have been performed to study potential
electron emission from free electron metals, but
the question of the competition between plasmon
excitation and Auger processes has not yet been
answered.
Most of the studies of potential plasmon excitation have considered ion impact on Al surfaces.
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0168-583X(03)01424-1
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P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 339–345
The difficulty in experiments on Al samples is the
strong overlap in the plasmon and Auger structures in the electron energy spectra. The Auger
neutralization structure has a maximum energy at
Eb ¼ I 0 2U, where I 0 is the ionization potential
of the parent atom shifted by the image interaction. This structure is thus separated in energy
from the plasmon by I 0 U Epl . Furthermore,
the distinction between the two structures can be
done by observing the different behavior of the
broadening of the Auger and the plasmon structure with changes in the ion velocity [7]. In Al,
the two structures can be distinguished only in
the case of incident Heþ ions [5,10], while for Arþ
ions no plasmon structure is observed [7], and for
Neþ no clear Auger structure has been resolved
[5,7,8].
In contrast, the spectra of electron emission
from Mg surfaces by slow ions carrying high
potential energy, such as Heþ and Neþ , show that
the plasmon decay and the Auger structures are
clearly separated in energy [1], due to the lower
energy of the plasmons of Mg.
Experiments on Mg surfaces appear, then, to be
better suited to study the role of plasmon decay in
potential electron emission. To this end, we measured energy distribution of electrons emitted from
Mg surfaces under the impact of slow Heþ ions.
Consistent with previous research [7,8], the
plasmon structure observed in the spectra induced
by slow Heþ ions is attributed to decay of multipole surface plasmons [9] excited by the potential
energy released by the neutralization at the surface
of incoming ions. Bulk plasmon excitation appears
at our highest impact energies.
We discuss methods to extract yields of electrons from plasmon decay from the experimental
spectra. A very simple data analysis procedure
allows disentangling the contribution given by
multipole plasmon decay from that by Auger
processes and from a low energy peak due to
electronic collision cascade.
2. Experiments
The experiments were performed in an ultra
high vacuum (base pressure 1 1010 Torr) sur-
face science system used in previous electron
emission studies [5], using a double-pass, cylindrical mirror, electron energy spectrometer. The
spectrometer was operated at constant pass energy
of 50 eV and a resolution of 0.2 eV. The energy
scale was calibrated to refer to the vacuum level of
the Mg sample [5]. The surface of the sample was
normal to the axis of the spectrometer and at 12°
with respect to the ion beam direction. Ions were
produced in an electron impact source, which was
operated at an electron energy of 58 eV. The
polycrystalline Mg surface was sputter cleaned by
4 keV Arþ ion bombardment and cleaning was
monitored by ion and electron induced Auger
electron spectroscopy.
3. Results
In the upper panel of Fig. 1, we report energy
distributions N ðEÞ of electrons emitted by the Mg
surface under the impact of 160–460 eV Heþ ions.
The spectra in Fig. 1 have been normalized so that
their area equals the electron emission yield ctot as
in [5], being consistent with those previous studies.
They show two structures labeled a and b, superimposed on a background peak of cascade electrons [4]. These structures appear clearer in the
derivative of the spectra shown in the lower panel
of Fig. 1.
The energy of the structure b is consistent with
the energy Eb ¼ I 0 2U (U ¼ 3:75 eV is the work
function for polycrystalline Mg) expected for the
high-energy edge of the spectrum of the electrons
emitted from the Mg surface by Auger neutralization. This attribution is confirmed by the
observed broadening of the electron energy distributions N ðEÞ with increasing projectile energy
(see inset in Fig. 1). This broadening is typical of
neutralization via Auger processes and results
from the atomic energy level shift near the surface
and incomplete adiabaticity due to ion motion
normal to the surface [13]. Furthermore, the spectra shown in Fig. 1 intersect at a ‘‘magical’’ point
[14], another characteristic of velocity broadened Auger spectra, which corresponds to the
minimum of the structure b observed in the derivative.
P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 339–345
He+ → Mg
+
He → M g
N(E) (e/eV)
b
N(E) (e/eV)
0.06
0.04
10
160
260
360
460
-3
10
-4
10
-5
341
eV
eV
eV
eV
0
5
6
7
8
9 5
0.4 keV (e)
(a)
0.4 keV
-1.2
-2
-0.8
10
12
14
16
18
-3
20
+
He → M g
0.02
16 0
26 0
36 0
46 0
a
b
-0.4
-5
eV
eV
eV
eV
0. 0
1 keV
(b)
(f)
1keV
-2
0
-1.2
dN/dE (10-3 e/eV2)
dN/dE (10 e/eV )
-3
-3
2
b
+
He → M g
160
260
360
460
a
-5
0
2
4
6
8
10
eV
eV
eV
eV
12
14
16
18
20
Kinetic Energy (eV)
Fig. 1. Top: Energy spectra N ðEÞ of electrons emitted by the
Mg surface by impact of 160–460 eV Heþ ions. Bottom: Derivative dN ðEÞ=dE that enhances the structure due to plasmon
decay and Auger neutralization. The derivatives are arbitrarily
displaced in the vertical scale for clarity. The structures labeled
a and b and approximately marked by vertical lines indicate
respectively the edge of the plasmon decay and Auger features.
The structure labeled a does not depend on the
ionization potential of the incoming ions [5]. Consistent with studies [7,8] on Al surfaces, structure a
is assigned to electron emission from decay of multipole surface plasmons, while bulk plasmon excitation appears at higher impact energy as shown in
Fig. 2.
The transition from multipole to bulk plasmon
excitation is shown in Fig. 2 Panels (a)–(d) of Fig.
2 report the plasmon dips observed in the derivative of the spectra revealed at growing incident ion
energies. At 260 eV incident energy, the plasmon
shoulder results in a minimum in the derivative at
Em 6:2 eV, i.e. 0.9 eV less than the energy of the
q ¼ 0 bulk plasmon structure appearing in the
-0.8
-5
-7
-0.4
-9
2.5keV (g)
(c)
-2.0
2.5keV
-3
-1.6
-7
-1.2
-10
-0.8
-14
-0.4
-3 (d)
4keV
(h)
4 keV
-2.0
-7
-1.6
-10
-1.2
-0.8
-14
-0.4
-17
5
6
7
8
9
5
0. 0
Energy (eV)
Fig. 2. (a–d) Derivative dN ðEÞ=dE showing the transition from
multipole surface plasmon (mSP) to bulk plasmon (BP) excitation. The vertical lines mark approximately the position of the
multipole and of the bulk plasmon. The lines drawn through
points indicate the polynomial secondary electrons background;
(e–h) Gaussian curve fits of the negative peaks obtained after
background subtraction.
electron excited spectrum [1]. This energy value is
consistent with the attribution to electron emission
from decay of multipole surface plasmons excited
at or above the surface by potential energy transfer
upon neutralization of incoming ions.
At an incident energy of 4.5 keV, the plasmon
structure appears at an energy that is consistent
with that of the Mg bulk plasmon. The spectra acquired at intermediate energies show the transition
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P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 339–345
from surface to bulk plasmon excitation. As the
energy of the incident ion is increased, we notice in
Fig. 2 that the structure due to bulk plasmon decay grows on the high-energy side of the surface
plasmon decay structure.
A BP
A SP
3
2
-3
A (10 e/eV)
4. Discussion
4
1
0
4
3
-3
R = ABP/γtot (10 /eV)
The task of obtaining information about the
electron yields for plasmon decay is rather difficult
due to the overlap of structures arising from several processes. Several procedures have been applied [15,16] to disentangle the plasmon structure
from the background due to other electron emission phenomena. Fig. 2 illustrates the application
of one this procedure [16], based on the analysis of
the derivative of the spectra, to the case of electron
emission by Heþ ion impact on Mg surfaces.
Panels (a)–(d) of Fig. 2 report examples of
background subtraction from the derivative of the
spectra. The background curve was obtained by
fitting two regions on both side of the plasmon
structure with a polynomial function. Panels (e)–
(h) report the negative peaks obtained after the
subtraction. We observe that at 460 eV incident
energy the structure is well reproduced by a
Gaussian curve centered at an energy below that
corresponding to a bulk plasmon decay structure
and assigned to decay of multipole surface plasmons. At higher incident energies, the structure is
not reproduced by only one curve, but by the superposition of two Gaussians of constant width
and position corresponding to the surface and the
bulk plasmon features. The areas ASP and ABP of
these Gaussians are shown as a function of incident ion energy in the upper panel of Fig. 3. Also
shown in the lower panel of Fig. 3 is the ratio
R ¼ ABP =ctot .
The analysis shown in Figs. 2 and 3 compares
well with similar results obtained in the case of
experiments of Neþ ion impact on Al surfaces [8],
showing that the procedure gives valuable insight
into the mechanisms for plasmon production in
free electron metals by slow ions. In [16], it has
been discussed that absolute electron yields from
surface and bulk plasmon decay could be obtained
by multiplying by a factor C ¼ 2EF =3 the areas of
2
+
He → Mg
1
0
0
1
2
3
4
5
Incident energy (keV)
Fig. 3. Top: Areas of the surface (ASP ) and bulk plasmon (ABP )
Gaussians described in Fig. 2 versus Heþ incident energy. The
lines through data points are used to guide the eye. Bottom:
ratio R ¼ ABP =ctot versus incident Heþ energy.
the corresponding Gaussians in Fig. 2. However,
we find that this value of the factor C is a good
approximation only for very narrow plasmon
width, while for plasmon structure whose width is
about 2 eV, like in Mg and Al, this value is underestimated by about 40%. Furthermore, as will
be shown later in this section, the particular choice
of the background curve affects the areas of the
plasmon structure in the derivative resulting in a
further strong underestimation. Therefore, to
study the relative contribution to electron emission
from plasmon decay and neutralization via Auger
P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 339–345
0.07
0
2
4
6
8
10
12
14
16
18
20
+
N(e) (e/eV)
460 eV He → Mg
0.04
Nk
NA
Np
0.00
0
-4
-3
2
dN/dE (10 e/eV )
processes, we adopted the procedure shown in
Fig. 3.
The energy distribution N ðEÞ of emitted electrons can be written as N ðEÞ ¼ T ðEÞN0 ðEÞ, where
N0 ðEÞ is the internal energy distribution, i.e. the
spectrum of electrons excited in the solid at energy
E above the vacuum level and T ðEÞ is the surface
transmission function, giving the probability for
an electron of excitation energy E to be transmitted through the surface. As shown in Fig. 1, electron cascade, plasmon decay and Auger electrons
contribute to N ðEÞ. Thus, we have N0 ðEÞ ¼
N0K ðEÞ þ N0p ðEÞ þ N0A ðEÞ, where N0K , N0p , N0A are
the internal energy distribution of electrons excited
respectively by the three processes. In the case of
experiments on Mg samples, the three emission
mechanisms are well separated in energy. Fig. 4
shows the attempt to reproduce the experimental spectrum N ðEÞ of electrons emitted by 460 eV
Heþ ion impact by modeling the three internal
distributions in the very simplest fashion and
by using a suitable choice for T ðEÞ as Eq. (16) in
[4].
As in [16], for N0p we considered the convolution of a parabolic density of states with a Lorentzian. We found that a width Cpl 1:7 eV,
which is consistent with the width of the plasmon
structures we observed in electron energy loss experiments, yielded a good reproduction of the experimental spectrum. The other adjustable
parameter in N0p is Em , the high-energy edge of the
plasmon decay structure. The value of Em we
found agrees with the expectation of multipole
plasmon decay (the multipole plasmon energy
is 0.9Evp , where Evp is the q ¼ 0 bulk plasmon
energy).
For N0A we considered the self-convolution of
a parabolic density of states to represent Auger
transitions involving two electrons in the Mg
conduction band and a hole in the incoming ion. A
Lorentzian broadening of a 0.5 eV was introduced in the convolution and the energy released
in neutralization, En , was varied to reproduce the
position of the structure, thus taking into account
the image energy shift. This simple model can reproduce the Auger structure well, except for the
high-energy tail. However, this discrepancy does
not introduce a significant error in the estimation
343
-8
-12
-16
0
2
4
6
8
10
12
14
16
18
20
Kinetic Energy (eV)
Fig. 4. Top: The energy spectrum of electrons ejected from Mg
by 460 Heþ eV ions is reproduced by the sum of three contributions: collision cascade (Nk ), plasmon decay (Np ) and Auger
neutralization (NAN ). Bottom: Derivative of the experimental
and the calculated spectra.
of the area of the structure, and is therefore neglected here to keep the description on the simplest
grounds.
For N0k ðEÞ, we found that a simple exponential
function of the type abE yielded a good reproduction of the cascade peak. Fig. 4 shows that the
experimental spectrum can be reproduced satisfactorily, allowing the estimation of the relative
contribution to electron emission due to plasmon
decay and Auger neutralization, i.e. the ratios
RSP ¼ cSP =ctot and RA ¼ cA =ctot , where cSP and cA
are the electron yields for the two process respectively. We find RSP ¼ 0:25 and RA ¼ 0:15. The
uncertainties in these values are estimated to be
about 15% by varying the relative weight of the
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P. Riccardi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 339–345
three calculated energy distributions in the reproduction of the experimental spectrum and also by
using other functional forms for N0k ðEÞ, as well as
for the surface transmission function T ðEÞ.
To further check the reliability of the analysis,
the lower panel of Fig. 4 shows that the procedure
gives also a good reproduction of the derivative of
the experimental spectrum, where the plasmon
feature is clearer. This observation confirms that
the plasmon excitation is needed to explain the
data and that its contribution to the electron
emission spectrum is properly estimated.
It is interesting to notice that the background
given by the derivative of the cascade peak is significantly different from that shown in Fig. 2.
Subtraction of this background from the derivative of the experimental spectrum results in the
negative peak shown in Fig. 5, which has an area
about twice larger than the subtracted peak shown
in panel (e) of Fig. 2. The subtracted structure in
Fig. 4 shows the Lorentzian broadening due to
finite plasmon lifetime, but it is also fairly well
-2.5
Subtracted plasmon dip
Lorentzian
Gaussian
dN/dE (10-3 e/eV2)
-2.0
-1.5
-1.0
-0.5
0.0
5
6
7
8
9
reproduced by a Gaussian distribution, thus justifying the use of Gaussian curves so far adopted
in Fig. 2 and in the literature [8,16]. The different
tailing of the two distributions is masked when the
background shown in Fig. 2 is subtracted from the
experimental data.
In conclusion, the results presented in this work
are consistent with the physical picture depicted in
previous studies of plasmon excitation in free
electron metals by slow ions. Low momentum bulk
plasmons are observed for incident energies in the
kinetic electron emission regime [4], in which
electron excitation is mostly determined by the
transfer of the kinetic energy of the projectile.
At lower energies, where electron emission is
mainly determined by the transfer of the potential
energy released when incoming ions neutralize, we
observe the excitation of multipole surface plasmons.
We have shown that existing methods of analysis of the derivatives of the experimental spectra
give valuable insight into the mechanisms for
plasmon excitation by slow ions, but require particular care in the attempt of extracting yields of
electron emission from plasmon decay. Therefore,
we applied a different procedure based on the
analysis of the spectra.
Our admittedly simplified data analysis cannot
fully account for the wealth of information contained in the spectra, which call for theoretical
investigation. Nevertheless, it strongly supports
the important conclusion that electron emission
from plasmon decay is more intense than that
from Auger neutralization. This results is in contrast with recent works [10–12], were plasmon excitation and decay was discussed as a minor
contribution to potential electron emission from
Al surfaces under ion irradiation [10–12], and
confirms the importance of potential plasmon excitation in ion neutralization at free electron metal
surfaces.
10
Energy (eV)
Fig. 5. Lorentzian and Gaussian fit of the negative plasmon dip
resulting after subtraction from the derivative of the experimental spectrum of the background given by the derivative of
the cascade peak shown in Fig. 3.
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