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W. Meissl:
"Response of insulator surfaces to a very slowly approaching highly charged ion";
Vortrag: 14th Intern. Conf. on the Physics of Highly Charged Ions (HCI-2008), University of Electro-Communications, Chofu, Tokyo/Japan (eingeladen); 01.09.2008; in: "Book of Abstracts, 14th Intern. Conf. on the Physics of Highly Charged Ions", Invited Progress Report (2008), S. 16.

Contrary to the interaction of slow highly charged ions with conducting targets, the physical
scenario of their impact on insulators is not very well understood by now. A deeper understanding
of this interaction is desirable as it would aid their employment as a gentle tool for surface
nanostructuring, where they have been recently shown to induce similar defect as swift (? GeV)
heavy ions, but at a much lower cost and without damaging the deeper layers of the target.
Upon impact on a solid surface the potential energy stored in slow highly charged ions
(HCI) is primarily deposited into the electronic system of the target. With respect to electron
emission, the charge mobility on these targets is limited; therefore the electron holes created during
the impact of a HCI might interfere with the incoming projectile and therefore alter the interaction
process ("trampoline-effect"?). Electron emission experiments were performed on insulating KBr,
LiF and CaF2 single crystal surfaces bombarded by slow highly charged xenon ions. We have
recently shown [1] that for somewhat faster ions (? keV/amu) impinging on insulators, the "hollow-
atom" decay process is by far not complete at the time of impact, as a strong sub-surface
contribution to the electron emission yield was found. More recent experiments with very slow
(down to 30 eV/amu) Xe HCI show a velocity dependence of the electron yield on the impact
velocity that clearly deviates from the case of metallic surfaces.
CaF2(111) surfaces that have been irradiated by slow highly charged ions have been
analyzed by atomic force microscopy (AFM). We have observed hillock-like topographic
nanostructures which are stable in air and non erasable by AFM scanning [2]. The number density
of surface structures is identical to the applied fluence, thus every individual ion creates one nano-
hillock. A sharp and well-defined threshold of potential energy is required for the onset of hillock
formation but neither the threshold nor the size of the structures strongly depend on the kinetic
energy of the projectiles [2,3]. We show that similar to the swift heavy ion case, the emission of
energetic electrons into the solid and the conversion of these electrons into lattice vibrations give
rise to a local melting of the impact region. Simulations of the dissipation of potential energy into
the target material on the basis of an extended classical over-the-barrier model have been performed
to facilitate the interpretation of the experimental findings [4].

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).