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 the mechanisms of molecular ejection from an irradiated molecular solid is investigated by large-scale molecular dynamics simulations. The results of simulations 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 surface is observed and the desorption yield is described by an Arrhenius-type dependence on the laser fluence. At fluences above the threshold, a collective 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 description of the fluence dependence of the total yield in simulations with different initial temperatures. Comparison of the simulation results for two pulse durations is performed to elucidate the differences in the ejection mechanisms in the regimes of thermal and stress confinement. We find that in the regime of stress confinement, high thermoelastic pressure can cause mechanical fracture/cavitation leading to energetically efficient ablation and ejection of large relatively cold chunks of material.