Physica Scripta. T92, 227^229, 2001
Mechanisms for Plasmon Production by Highly Charged Neon Ions
Interacting with an Al Surface
N. Stolterfoht1, J. H. Bremer1, V. Ho¡mann1, D. Niemann1, M. RÎsler1 and R. Baragiola2
1
2
Hahn-Meitner-Institut Berlin GmbH, Glienickerstr. 100, D-14109 Berlin, Germany
Laboratory for Atomic and Surface Physics, University of Virginia, Engineering Physics, Charlottesville, VA 22901, USA
Received July 31, 2000; accepted September 20, 2000
pacs ref: 71.45.GM, 73.20.Mf, 34.50.Dy
Abstract
4‡
Low-energy electrons ejected by 1^4 keV Ne ion impact on an Al surface
were measured. Spectral structures found near 11 eV were attributed to
the decay of plasmons induced by potential energy of the projectile. The data
were used to determine absolute values for electron yields from the plasmon
decay, which were primarily studied as a function of the incidence angle
of the Ne4‡ projectile. Strong variations of the plasmon yield are observed
when the angle of incidence reaches that of specular re£ection indicating signi¢cant changes in the contributions to plasmon production from below
and above surface.
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 free-electron 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 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 plasmon-assisted capture
where the transfer of potential energy from the projectile
produces a plasmon [5^11]. Particular attention has been
focused on slow heavy ions incident on a surface with a high
charge state [7,12^14]. 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 [15].
Above the surface, highly charged ions strongly attract several electrons, which are resonantly captured into high
Rydberg states whereas inner shells remain empty. Thus,
the projectiles evolve into hollow atoms 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 [16], the decay of both
bulk plasmons [6,7,13] 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 [10].
# Physica Scripta 2001
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. It is shown that the use
of highly charged projectiles, give rise to characteristic
properties in the scenario of plasmon production.
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 [17]. Details of
the experimental method have been presented before
[12,13,18]. A beam of Ne4‡ ions was collimated to a diameter of about 1 mm and directed onto a clean Al(111) target.
The pressure in the chamber was a few 10^10 mbar. The emission of electrons from the target was measured using an
electrostatic parallel-plate spectrometer. The spectrometer
e¤ciency and the ion current were determined [18] so that
absolute values for electron emission yield could be
measured. 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; O† ˆ
dY =dedO measured in our group are given in Ref. [7,12,13].
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.
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]. To enhance the visibility of the plasmon
structures, which are superimposed on an intense background from other processes, it is common practice to
di¡erentiate the measured electron intensities N…e; O†.
Results for the derivative dN=de are given in Fig. 1 for 2 keV
Ne4‡ impact on Al. The graphs labeled (a), (b), and (c) on the
left-hand side show dN=de for incidence angles of c ˆ 1 ; 8 ,
and 18 , 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. 1(a).
Each dN=de curve clearly shows a structure near 11 eV
which is attributed to plasmon decay [7,13]. As discussed
in detail previously [13], 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 below
Physica Scripta T92
228
N. Stolterfoht et al.
Fig. 1. Derivative of electron yields measured for 2 keV Ne4‡ impact on Al as
a function of the electron energy. In (a), (b), and (c) data are given for the
incidence angles c ˆ 1 , 8 and 18, respectively, as shown in the left hand
side graphs. On the right hand side the plasmon structures are shown after
subtraction of the background.
the plasmon structure by a second-order polynomial (given
in Fig. 1(a^c) as dashed lines). 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. Furthermore, it is found
that the dip intensity for the intermediate angle of c ˆ 8
is smaller than those for c ˆ 1 and 18 .
For further analysis of the data we integrated the peak
structures after background subtraction. As shown in detail
previously, the electron yields from plasmon decay can be
obtained by multiplying the integrated results by 6.3 eV [19].
In Fig. 2(a^c) absolute values of the electron yield dY =dO
are plotted for projectile energies of 1, 2, and 4 keV,
respectively. The dY =dO curves show strong variations with
the incidence angle c. 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 work [5^7] we consider excitation by potential
energy transfer as a primary 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 may be
Physica Scripta T92
Fig. 2. Plasmon yield from Al for Ne4‡ impact as a function of the incidence
angle c. 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.
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 [20]. However,
energetic electrons produced directly in collisions as well
as Auger electrons may excite plasmons when traveling
through the solid [13,21].
For a better understanding of the data in Fig. 2, we consider two regions relevant for plasmon production: 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. 1 the 11 eV structure observed for c ˆ 1 is signi¢cantly larger in energy than Al surface plasmons observed
at 6.4 eV [4]. However, as emphasized previously [8,10],
the latter energy refers to zero momentum plasmons,
whereas surface plasmons with larger momentum may produce electrons of energies as high as 11 eV. An increase
of the momentum transfer with increasing energy would
be consistent with the shift of the plasmon dip observed
in Fig. 1. Moreover, high momentum transfer may result
in the production of surface plasmons of higher multipoles
[10]. In addition, similar to He*[16] one should consider
the Auger capture into Ne*, which also gives rise to electrons
with energies near 11 eV for Al.
# Physica Scripta 2001
Mechanisms for Plasmon Production by Highly Charged Neon Ions Interacting with an Al Surface
In the following, the electron yields due to above- and
below-surface processes are denoted dYA =dO and dYB =
dO, respectively. These yields are governed by the re£ection
probability pR of the projectile at the surface, which strongly
depends on the incidence angle c. In a simple model we set
dYB
ˆ B…1
dO
pR †
…1†
where B is a constant assuming that the ions penetrating into
the solid produce bulk plasmons. Similarly, we set
dYA
ˆ ApR …1
dO
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 decay 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 †, where u is a constant velocity and
vz ˆ v sin c is the vertical velocity of the projectile. It can
readily be shown that u 2dGL 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
(exhibited in the electron spectra of, e.g., Refs. [6,13]).
Finally, the sum dYA =dO ‡ dYB =dO was ¢tted to the
experimental data treating A and B as adjustable parameters
for each incident energy. The results are plotted in Fig. 2
where apart from the sum (solid lines) also the individual
yields dYA =dO (dashed lines) and dYB =dO (dashed dotted
lines) are given. It is seen that the theoretical data compare
well with experiment 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 below surface contributions to the
plasmon yield. We note that these contributions are
governed by the parameters A and B, respectively, which
# Physica Scripta 2001
229
are obtained as asymptotic electron yields A ˆ dYA …c !
0†=dO and B ˆ dYB …c 0†=dO. It is found that the parameters A and B have opposite energy dependencies. It
can readily be veri¢ed from Fig. 2 that A decreases slightly,
whereas B increases signi¢cantly with increasing projectile
energy. This appears to con¢rm potential energy e¡ects
to be responsible for the above-surface processes, whereas
the below-surfaces processes are expected to be in£uenced
by kinetic energy e¡ects. In view of this ¢nding, future work
is needed to verify the opposite energy dependencies of the
contributions to plasmon production above and below
the surface.
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