Target-vapour interaction and atomic collisions in pulsed laser ablation

Numerical solution of the Boltzmann equation with a relaxation collision term is used to study gas-dynamic flows formed under nanosecond pulsed laser ablation. Atoms ejected from the surface of a target are assumed to have a 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. Atomic collisions in the ablation plume orient atoms towards the surface normal and speed up the plume expansion from the target. Atoms backscattered in the gas phase, stick to the target surface and cause back condensation of the vapour at later stages. When the mean free path is much less than the plume dimension, a Knudsen layer, a hydrodynamic flow region, and a low-density tail may be distinguished in the gas phase. The present numerical simulation is in good agreement with the analytical quasi-steady Mott-Smith approach to the Knudsen layer in the case of evaporation and at the early stages of condensation. Comparison with experiment reveals that the model underestimates both the width of the ablated material angular distribution and the amount of high-energy atoms. The difference increases with the laser fluence and may be caused by lateral expansion of the plume, by vapour acceleration due to laser radiation absorption or probably by non-thermal evaporation effects. (author)