Microscopic mechanisms of laser spallation and ablation of metal targets from large-scale molecular dynamics simulations

The microscopic mechanisms of femtosecond laser ablation of an Al target are investigated in large-scale massively parallel atomistic simulations performed with a computational model combining classical molecular dynamics technique with a continuum description of the laser excitation and subsequent relaxation of conduction band electrons. The relatively large lateral size of the computational systems used in the simulations enables a detailed analysis of the evolution of multiple voids generated in a sub-surface region of the irradiated target in the spallation regime, when the material ejection is driven by the relaxation of laser-induced stresses. The nucleation, growth, and coalescence of voids take place within a broad (100 nm) region of the target, leading to the formation of a transient foamy structure of interconnected liquid regions and eventual separation (or spallation) of a thin liquid layer from the bulk of the target. The thickness of the spalled layer is decreasing from the maximum of 50 nm while the temperature and ejection velocity are increasing with increasing fluence. At a fluence of 2.5 times the spallation threshold, the top part of the target reaches the conditions for an explosive decomposition into vapor and small clusters/droplets, marking the transition to the phase explosion regime of laser ablation. This transition is signified by a change in the composition of the ablation plume from large liquid droplets to a mixture of vapor-phase atoms and clusters/droplets of different sizes. The clusters of different sizes are spatially segregated in the expanding ablation plume, where small/medium size clusters present in the middle of the plume are followed by slower (velocities of less than 3 km/s) large droplets consisting of more than 10,000 atoms. The similarity of some of the characteristics of laser ablation of Al targets (e.g., evolution of voids in the spallation regime and cluster size distributions in the phase explosion regime) to the ones observed in earlier simulations performed for different target materials points to the common mechanical and thermodynamic origins of the underlying processes.