Surface nanostructures induced by slow highly charged ions

Surface modifications induced by the irradiation with ions have been investigated for long time. Depending on their energy the ions create a crater resulting from nuclear or electronic sputtering or can induce the formation of hillocks at high kinetic energies. For highly charged ions (HCI) the situation is more complex. In addition to their kinetic energy HCI posses also potential energy which is the sum of the ionization energies to create them. This large amount of potential energy is released when interacting with solids and deposited into a small volume close to the surface. The resulting high excitation in the surface can thus induce various modifications [1]. Recently, it has been shown that hillock structures are formed on CaF2 by HCI due to a localized phase transition [2].
In the case of the alkali halide crystal KBr we observed the formation of nanostructures resulting by desorption of surface atoms from a single ion impact site [3]. For high enough charge states each ion produces a mono-atomic deep pit with a diameter of 10-30 nm (depending on the charge state) on the atomically flat surface. The desorption of such a high amount of material can not be induced by kinetic sputtering alone, which dominates in this kinetic energy regime, but is induced by the excitation due to the potential energy. For a kinetic energy of 40 keV a threshold in the potential energy of the HCI is found for the formation of these structures between Xe15+ and Xe20+ around 3 keV. Above this threshold the volume of the pits and therewith the potential sputtering yield exhibits a linear dependence on the potential energy. However, the kinetic energy also plays an important role. For higher kinetic energies the threshold shifts to lower values of the potential energy.