Influence of atomic collisions in vapour phase on pulsed laser ablation

To investigate the role of atomic collisions in pulsed laser ablation, the one-dimensional Boltzmann equation with the Bhatnagar-Gross-Krook collision term is solved. Atoms ejected from the surface of a target are assumed to have the Maxwell velocity distribution corresponding to the surface temperature and the saturated vapour pressure. The surface temperature is obtained from a transient heat transfer equation in the condensed phase. The model describes both the thermal evaporation regime without collisions at low laser fluences and dense ablation plumes at high fluences. When the mean free path is much lower than the plume dimension, the Knudsen layer and the hydrodynamic flow region may be distinguished in the gas. The hydrodynamic parameters formed behind the Knudsen layer an in good agreement with the Anisimov-Knight jump conditions till the target is evaporated. When the laser pulse is over, a back vapour condensation may be appreciable. (C) 2000 Elsevier Science B.V. All rights reserved.