Mechanisms of laser ablation from molecular dynamics simulations: dependence on the initial temperature and pulse duration

The effect of the initial sample temperature and laser pulse duration on th e mechanisms of molecular ejection from an irradiated molecular solid is in vestigated by large-scale molecular dynamics simulations. The results of si mulations performed for two initial temperatures are found to be consistent with the notion of two distinct regimes of molecular ejection separated by a threshold fluence. At low laser fluences, thermal desorption from the su rface is observed and the desorption yield is described by an Arrhenius-typ e dependence on the laser fluence. At fluences above the threshold, a colle ctive multilayer ejection or ablation occurs and the ablation depth follows a critical density of the deposited energy. The same activation energy for desorption and critical energy density for ablation provide a good descrip tion of the fluence dependence of the total yield in simulations with diffe rent initial temperatures. Comparison of the simulation results for two pul se durations is performed to elucidate the differences in the ejection mech anisms in the regimes of thermal and stress confinement. We find that in th e regime of stress confinement, high thermoelastic pressure can cause mecha nical fracture/cavitation leading to energetically efficient ablation and e jection of large relatively cold chunks of material.