One of the surprising aspects of the impact of highly charged ions HCI^q+ (as Ar^8-9+, Xe⁺⁴³ ~ Xe⁺⁴⁵) has been the observation of concave nanohillocks [1]. They have been observed using noncontact Atomic Force Microscopy (AFM) and Scanning Tunnelling Microscopy (STM) on a graphite (highly oriented pyrolytic graphite: HOPG) surfaces, In the case of HOPG, the height of those hillocks are as low as 0.2-0.3 nm. On the other hand, the lateral sizes increases slowly from 0.4 nm up to 1.2 nm if used the potential energy of Ar^q+ (0.02à3keV), then it turns to further increases steeply from 1.2 nm up to 7.0 nm if used Xe ^q+ (3 à90 keV). Similar hillocks are also formed in CaF₂, SiO₂, Al₂O₃.

For the mechanism of hillock formation, a “modified inelastic thermal spike model” is qualitatively accepted [1], where the electronic excitation gives rise to electron phonon interaction and lead surface modification. Nevertheless, consideration of the neutralization process of a HCI, many electrons can be captured by the HCI with kinetic energies leaving many cations in a target within a small volume [2]. Here we propose a new model for the interaction between excited-electron and localized-phonon and study how the hillock will be formed.

[1] F. Aumayr, A.S. El-Said, W. Meissl: Bucl. Instr. Meth. B 266 (2008) 2729–2735.

[2] Y. Yamamura, S. T. Nakagawa, and H. Tawara: Nucl. Instr. Meth. B 98 (2005) 400-406.