Ion - surf ace interaction is a topic, which attracted a lot interest due to both its promising applications and the concerned interesting basic phenomena. Recently, slow highly charged ions (HCI) showed their high effectiveness in creating nanostructures on various surfaces [1 - 4]. The formed nanostructures depend highly on the selected material as well as the type of the used ions. Here we report on the modifications induced by HCI in BaF 2 and CaF 2 surfaces. Despite the fact that both materials belong to alkaline - ear th fluorides with the same crystalline structure, their response to HCI - Figure 1 . The near - field intensity distributions of the TE mode measured by the end - face coupling @ 633 nm and the Raman shift . T U E S D A Y Tuesd ay - Posters 137 induced nanostructures is different [5, 6]. The observed nanohillocks and etch pits in both surfaces will be discussed in terms of the dependence on kinetic and potential energy of the used HCI. Furthermore, a comparison with the surface modifications by swift heavy ions (SHI) in both materials is conducted in order to get more insight toward a better understanding of the formation mechanisms of the nanostructures [7].