Energy distribution of ions in plasma formed by laser ablation of metallicNb and Ta targets

The laser plasma plume expansion induced by UV nanosecond irradiation from metallic Nb and Ta targets has been studied using Langmuir probe time-of-fl ight measurements. The ion current shape was measured as a function of the laser radiation flux at various probe-to-target distances. The ion velocity distribution and the ion energy spectra have been determined by measuring the time of flight of ions reaching the Langmuir probe. A multi-mode veloci ty distribution of ions has been revealed. The dependence of the amplitudes of the plume energy components on the plume scatter time and on the laser radiation flux has been investigated. The experimental time-of-flight data have been compared with a Maxwell velocity distribution. The threshold valu es of the laser intensity were determined for different distribution modes in the ion energy spectrum. (C) 2000 Elsevier Science Ltd. All rights reser ved.