On the relaxation of electron energy distribution function in LIBS plasmas

The relaxation of electron energy distribution function (eedf) in excited atomic nitrogen plasma (N, N+, e) is numerically studied by solving an appropriate time-dependent Boltzmann equation including elastic, inelastic, superelastic and Coulomb collisions. Conditions typically met under laser breakdown plasma spectroscopy (LIBS) and characterized by large ionization degrees are considered. The results show that the relaxation of eedf after an abrupt decrease of temperature goes through non-Maxwell distribution functions to the final Maxwell distribution function. It is shown that in conditions characteristic for LIBS plasma the deviation of the eedf from the maxwellian one is mainly caused by superelastic collisions. For atmospheric plasmas characteristic equilibration times span μs and sub-μs regions.