Molecular-dynamics simulation of prompt sputtering of a molecular solid at high excitation densities

Prompt sputtering from a weakly bound, amorphous solid at high excitation densities is simulated with use of molecular dynamics in order to understand the ejection of a volume of material by fast heavy ions–the process thought to be responsible for ejection of whole organic molecules from a solid. Ejection of diatomic molecules from a solid was calculated as a response to a large amount of energy deposited in the vibrational excitation of those molecules within a narrow cylinder: the incident ion’s ‘‘track.’’ Calculations were performed for two modes of vibrational excitation and for samples with different geometries, number densities, cohesive energies, thicknesses, and boundary conditions. For high excitation densities, rapid (∼${10}^{\mathrm{-}12}$ sec) energy transfer takes place between the internal energy and the center-of-mass motion, resulting in molecular ejection. The yield for prompt sputtering is found to be independent of the molecular mass for a given excitation density and has a steeper than cubic dependence on the energy density deposited in the solid for the energy densities studied, exhibiting a ‘‘threshold’’ at the lowest excitation densities. The scaling with excitation density found is close to that calculated earlier for structureless particles and to the pressure-pulse model but disagrees with the standard ‘‘shock’’ models.