Surface Modification on KBr(001) with Slow Highly Charged Ions in High Fluence and High Potential Energy Regime

Highly charged ions (HCI) have an unique property, their potential energy, which can induce modifications on surfaces on the nano-scale without a significant bulk damage. The potential energy, defined as the sum of the binding energies of all missing electrons, can exceed 100 keV, e.g. for Xe50+ or Au60+. This amount of energy is mainly released by the emission of electrons with energies of several 10 eV as described by the classical over the barrier model. The release of the HCI’s potential energy as emission of electrons right in front of the surface or in the first nanometers of the solid leads to a strong interaction with the electronic system of the solid. Especially in ionic crystals like KBr many electron-hole pairs are created near the impact site of the HCI. Due to a strong electronphonon coupling in ionic crystals these excitons become self-trapped and can either recombine or decay into so called colour centres. In KBr F and H-centres are created. H-centres are defined as an interstitial molecular halide atom and a F-center is an electron at an anion site. F-centres, which were produced near the surface, can recombine with surface ions (K+) and lead to their neutralisation and desorption. Also a Br-atom desorption occurs when the H-centre recombines with the surface.
Recently nano-pits on KBr surfaces induced by single ion impacts were investigated and a potential as well as a kinetic energy-threshold for their formation was found. The depths of the pits produced by a single ion impact was found to be one atomic monolayer for Xe charge states up to q = 40 and various kinetic energies. Low charge state (q>10) irradiations, in contrast, show pits as well, but only for much higher fluences (several 10^13 cm^-2). We present studies of high fluence irradiations with highly (q>10) and lowly (q<10) charged Xe ions. For high charge states a deepening of the pits (with increasing fluence) was found, while for low charge state irradiations a depletion region close to low coordinated sites like step edges has been observed. For irradiations with highest charges states, i.e. Au54+ ions and low fluences (<10^10 cm^-2) pit structures could be observed with double monolayer depths for single impacts. This indicates a second energy-threshold for the desorption of the second monolayer by a single ion impact. A qualitative model will be discussed to describe and identify the observed potential and kinetic energy effects.