Comment on "Monte Carlo simulation of the laser-induced plasma plume expansion under vacuum: Comparison with experiments" [J. Appl. Phys. 83, 5075 (1998)]

An attempt to use direct simulation Monte Carlo (DSMC) method for pulsed laser ablation at laser fluence as high as 17 J/cm is analyzed. Under these conditions, however, the evaporation flux is very high which results in a very high laser plume density. Therefore, both collision rate and collisional zone near the target are too large to effectively use a conventional DSMC technique whose possibilities are always limited by finite computer storage and available computation time. Nevertheless, the authors of the considered paper claim to propose a new approach to overcome the restrictions of DSMC. We found that the main idea of the approach is the plume reduction in space. However, according to the dimension theory, under this transformation one should preserve the Knudsen number, that defines the degree of rarefaction, and, hence, the number of cells to be used in DSMC. Therefore, the number density of the plume should be increased and particle collisions should be calculated with higher frequency than in the case without space reduction. In our opinion, this important point was neglected by the authors, the collision frequency was not increased, and the number of cells was not properly chosen. As a result, the proposed approach does not break the DSMC restrictions, and the results obtained do not correspond to the experiments with high evaporation flux. © 1999 American Institute of Physics.