The role of the photochemical fragmentation in laser ablation: a molecular dynamics study

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Despite numerous studies, the mechanistic understanding of the role of the photochemical processes and their coupling with the thermal processes in UV laser ablation is still far from being complete. In this work, the effects of the photochemical reactions on the laser ablation mechanism are delineated based on the results of molecular dynamics simulations of 248 nm laser irradiation of solid chlorobenzene. Photochemical reactions are found to release additional energy into the irradiated sample and decrease the average cohesive energy, therefore decreasing the value of the ablation threshold. The yield of emitted fragments becomes significant only above the ablation threshold. Below the ablation threshold only the most volatile photoproduct, HCl, is ejected in very small amounts, whereas the remainder of photoproducts are trapped inside the sample. Results of the simulations are in a good qualitative agreement with experimental data on the ejection of photoproducts in the laser ablation of chlorobenzene.