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 tempera
ture 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 las
er 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 hydr
odynamic flow region may be distinguished in the gas. The hydrodynamic para
meters formed behind the Knudsen layer an in good agreement with the Anisim
ov-Knight jump conditions till the target is evaporated. When the laser pul
se is over, a back vapour condensation may be appreciable. (C) 2000 Elsevie
r Science B.V. All rights reserved.