Molecular dynamics simulation study of the fluence dependence of particle yield and plume composition in laser desorption and ablation of organic solids

Two distinct regimes of molecular ejection separated by a well-defined thre shold fluence are observed in molecular dynamics simulation of pulsed laser irradiation of an organic solid. At fluences above the threshold a collect ive multilayer ejection or ablation occurs where large liquid droplets are ejected and the total yield of the ablated material follows a critical volu me density of the deposited energy. Below threshold thermal desorption from the surface is observed and the desorption yield has an Arrhenius-type dep endence on the laser fluence. The yield of monomers does not have a step in crease at the threshold and thus deceptively does not give a straightforwar d interpretation of the ejection mechanisms. (C) 1999 American Institute of Physics. [S0003-6951(99)01909-9].