Microscopic mechanisms of short pulse laser spallation of molecular solids

The mechanisms of photomechanical spallation are investigated in a large-scale MD simulation of laser interaction with a molecular target performed in an irradiation regime of inertial stress confinement. The relaxation of laser-induced thermoelastic stresses is found to be responsible for the nucleation, growth, and coalescence of voids in a broad sub-surface region of the irradiated target. The depth of the region subjected to void evolution is defined by the competition between the evolving tensile stresses and thermal softening of the material due to the laser heating. The initial void volume distribution obtained in the simulation of laser spallation can be well described by a power law. A similar volume distribution is obtained in a series of simulations of uniaxial expansion of the same molecular system performed at a strain rate and temperature realized in the irradiated target. Spatial and time evolution of the laser-induced pressure predicted in the MD simulation of laser spallation is related to the results of an integration of a thermoelastic wave equation. The scope of applicability of the continuum calculations is discussed.