A Hybrid MD-DSMC Model of Picosecond Laser Ablation and Desorption

A two-stage computational model of the evolution of a plume generated by laser ablation of an organic solid is presented and discussed. The first stage of the laser ablation involves laser coupling to the target and ejection of the plume and is described by molecular dynamics (MD) simulations. The following stage of a long-term expansion of the ejected plume is modeled by the direct simulation Monte Carlo (DSMC) method. The results of the MD simulations demonstrate that the physical mechanism of material ejection at sufficiently high laser fluences is a phase explosion of the overheated material followed by a homogeneous decomposition of the expanding plume into a mixture of liquid droplets (molecular clusters) and gas phase molecules. The extremely low proportion of large-size clusters hinders both statistical description of their parameters from the results of MD simulations and the following representation of each cluster size as a separate species, as required in the conventional DSMC. Therefore, a new computational scheme, which treats the size of large clusters as a random variable, is developed. The results of the hybrid model demonstrate that even for low laser fluences and short pulse duration, the evolution of the plume differs considerably from that predicted by pure thermal desorption models. For high fluences, the phase explosion of the target material and intensive processes of particle interactions within the plume are responsible for dramatic changes in the plume evolution as compared to that at low fluences.