1. State Key Lab for Materials Modification by Laser, Electron and Ion Beams, Dept. of Physics, Dalian University of Technology, Dalian 116024, P. R. China
2. Hubei Huaqiang Chemical Co., Yichang 443003, P. R. China
3. Lecce University, Physics Department, Istituto Nazionaie Fisica della Materia, PO Box 193, 73100 Lecce, Italy