[Zurück]

W. Meissl, D. Winklehner, R. Heller, S. Facsko, W. Möller, F. Aumayr:
"Electron emission from insulators bombarded with very slow highly charged ions";
Poster: 14th Intern. Conf. on the Physics of Highly Charged Ions (HCI-2008), University of Electro-Communications, Chofu, Tokyo/Japan; 02.09.2008; in: "Book of Abstracts, 14th Intern. Conf. on the Physics of Highly Charged Ions", (2008), S. 124.

Electron emission induced by impact of slow highly charged ions (HCI) on metallic surfaces has
been studied extensively over the past 15 years [1]. The "classical-over-the-barrier" (COB) model
[2] was very successful in modelling the formation of so-called "hollow atoms" in front of the
surface and the subsequent emission of electrons due to Auger and other processes. Because of the
finite hole mobility and differences in image charge potentials the response of insulator surfaces to
slow HCI impact, however, is less well understood. We have recently used an electron statistics
(ES) detector [3] to measure yields and number statistics of electrons emitted from (insulating)
LiF(001) and CaF2(111) surfaces bombarded by moderately slow (v < 1 a.u.) Arq+ (q <= 18+), Xeq+
(q <= 50+), and Hgq+ (q <= 68+) projectile ions under various impact angles [4]. The results show a
clear deviation in the behaviour of potential electron emission as compared to metallic surfaces in
this velocity regime. This deviation is explained by a strong sub-surface electron emission
contribution from the insulating targets.

In this work we present electron yields from very slow Xeq+ impact on clean, single crystalline
KBr(001), LiF(001) and CaF2(111) surfaces, all of which have recently found to be susceptible to
nanostructuring by single slow HCI impact [5] [6]. Charge states were varied between q = 10 and
44. Moderate heating of the targets was applied to counter any macroscopic charge-up during ion
irradiation. We have constructed a new ES setup that was implemented into the deceleration station
of the EBIT at the Forschungszentrum Dresden. Applying multiple electrical potentials, we first
bend the ion beam (up to 70°) to hit a tilted target under normal incidence and then - with the same
electrical field - collect close to 100% of the electrons emitted during the impact event. We have
determined the true ion beam energy and its energy spread by applying a retarding field on the
electron repeller aperture at the entrance of the chamber while monitoring the number of ions
hitting the target. With this setup, we were able to measure electron yields for impact energies of
less than 2 eV/amu up to 1.5 keV/amu, where a good agreement with previous data [4] for higher
impat velocities was found. For the first time, electron yields from insulators have been obtained in
the velocity regime close to the image-charge acceleration limit.

This work has been supported by Austrian Science Foundation FWF (P17449-N02) and by the
European Project RII3\#026015. Transnational access to the Rossendorf ion beam facilities was
provided through AIM (EU contract no. 025646).