In¯uence 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 ¯uences and dense ablation plumes at high ¯uences. When the mean free path is much lower than the plume dimension, the Knudsen layer and the hydrodynamic ¯ow region may be distinguished in the gas. The hydrodynamic parameters formed behind the Knudsen layer are 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. # 2000 Elsevier Science B.V. All rights reserved