1. Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via R. Cozzi 55, I-20125 Milano, Italy
2. CNR-IOM, Laboratorio TASC, S.S. 14 Km 163.5, Basovizza, 34149 Trieste, Italy
3. Department of Physics, University of Trieste, via A. Valerio 2, 34127 Trieste, Italy
4. Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China