[Zurück]

K. Dobes, E. Bodewits, G. Kowarik, R. Hoeckstra, F. Aumayr:
"Electron emission due to impact of highly charged ions on C₆₀ covered gold surfaces and HOPG";
Poster: 19th International Workshop on Inelastic Ion-Surface Collisions (IISC-19), Frauenchiemsee/Germany; 18.09.2012; in: "Book of Abstracts, 19th International Workshop on Inelastic Ion-Surface Collisions (IISC-19)", (2012), S. 35.

1. INTRODUCTION
Electron emission as a result of the interaction of highly
charged ions (HCI) with solid surfaces is of substantial
interest both with regard to fundamental research as well as
technical applications such as controlled nanostructuring
of surfaces or plasma surface interaction in e.g.
thermonuclear fusion devices. The emission of electrons
during the impact of a HCI on a solid surface is generally
divided into two different regimes, i.e. kinetic emission
(KE) and potential emission (PE). The former process is
driven by the kinetic energy of the impinging projectile,
while the latter is induced by the potential energy stored in
a highly charged ion Zq+ due the removal of q electrons. A
widely accepted model that describes the dissipation of the
large amounts of potential energy, which are carried by a
HCI, at the surface is the so-called hollow atom scenario
[1,2]. It describes the neutralization and relaxation of the
HCI upon surface impact. This process is not only governed
by the potential energy of the projectile, i.e. its charge state
q, but also depends on the electronic structure of the
surface, the work function and electron transport properties
of the target material [1].
To understand the influence of these properties on the
electron emission yield and the hollow atom decay in
greater detail, we studied and compared ion-induced
electron yields from different target materials with different
work functions. We investigated clean Au(111), a gold
surface covered with 1 - 5 monolayer thin films of C60 and
highly ordered pyrolytic graphite (HOPG) under the impact
of highly charged Ar and Xe ions at different impact angles
and energies.
2. EXPERIMENTAL SETUP
Experiments were performed at the setup IISIS at KVI
Groningen [3]. The Au and HOPG target respectively were
mounted within an UHV chamber at a base pressure of the
order of 10-10 mbar. The Au surface was cleaned by cycles of
sputtering and annealing. The HOPG sample was cleaved
with a scotch tape before it was transferred to the vacuum
chamber. Thin films of C60 were deposited onto the Au
sample by means of an Omicron EFM3 evaporator. Single
monolayers of C60 were deposited by first determining the
deposition rate with a quartz crystal microbalance and then
exposing the sample to the C60 beam for a corresponding
time interval.
Arq+ ions (q = 4, 6 - 13) and Xeq+ ions (q = 10, 12, 14, 16, 18,
20, 22, 24, 26, 28) were extracted from a 14 GHz ECR ion
source. The Ar ion energies ranged from 3.9 keV up to 91
keV. The impinging Xe ions were in an energy range from
7.2 keV to 328 keV.
The electron statistics detector [4] is mounted under 90°
with respect to the incoming ion beam. It is an energy
sensitive, passivated implanted planar silicon (PIPS)
detector. Secondary electrons, which are emitted in an ionsurface
collision event, are collected by a set of six different
electrodes surrounding the target and are then accelerated
towards the detector, which is biased to +30 keV. The
number of electrons produced in a single ion impact event
is determined by pulse height analysis. From this the
electron number statistics is obtained in addition to the
mean number of emitted electrons per incident projectile
ion.
3. RESULTS
When comparing secondary electron yields from Au and Au
covered with 1 - 5 ML of C60, an increase in electron yield is
found. This increase in electron yield is well described by
an exponential gain function and is virtually independent
of the potential energy of the projectiles between 0.5 and 10
keV. It saturates at 35%, when five monolayers of C60 are
evaporated on the surface [5]. Also for a clean HOPG surface
a higher yield is found as compared to a clean Au surface. A
detailed comparison of the results obtained on the different
targets will be presented and possible scenarios will be
discussed that are able to explain this increase in electron
yield.