Multi-mode velocity distribution of ions in UV laser-induced plasma plumes

Plasmas induced by UV laser ablation were studied and analysed using time-of-flight measurements. A KrF laser beam of 23ns full width at half maximum time duration was focused on Si and Nb targets with moderate laser irradiance of 108W/cm2. Suitable theoretical expressions were derived for fitting the recorded ion current under the assumption of a oshiftedo Maxwell-Boltzmann velocity distribution in order to estimate important parameters of the plasma, such as the ion temperature. Furthermore, deconvolution of time-resolved ion current signals, taking into account a multi-mode velocity distribution of ions, has revealed that the contribution of the different ion charge states induces an ambipolar electric field which influences the expanding plasma into regions with thermal and accelerated ions.