Phase transition at low ̄uences in laser desorption of organic solids: a molecular dynamics study

Molecular dynamics of a short pulse laser irradiation of a molecular crystal are performed to investigate the dependence of the mechanism of ejection on laser ̄uence. We ®nd that at low laser ̄uences molecules are desorbed from a thin surface layer of solid sample. An increase in the laser ̄uence leads to the melting of the surface region and desorption occurs from an overheated melted state. An additional increase of the ̄uence leads to the ablation when the laser-induced pressure and phase explosion of the overheated liquid drives a collective ejection of a signi®cant volume of irradiated material. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 02.70.Ns; 61.80.Az