Molecular dynamics calculations were performed to describe vibrational to translational energy transfer processes leading to desorption in a low cohesive energy solid excited by a laser pulse. In this study solid oxygen crystals were vibrationally excited and the redistribution of energy in the solid was followed for several nanoseconds. In a closed system, representing bulk processes, the energy transfer to the lattice occurred slowly at first while the vibrational modes equilibrated rapidly. This was followed by a short period of very rapid energy transfer to the lattice and full equilibration. The anharmonic nature of the O 2 potential, the density of excitation, and the lattice structure were identified as the factors determining the rate of internal vibration to lattice energy transfer. In addition, melting, leading to molecular diffusion, was shown to lead to very rapid (catastrophic) equilibration in the closed system. The addition of an interface with the vacuum allowed desorption/ablation. Such a sample could expand, dramatically slowing the equilibration process. The vibrational energy excitation rate, which changes with laser pulse length, was shown to affect both the conversion from the internal vibrational to lattice heating and the desorption efficiency. The competition between energy transfer to the lattice and material expansion was such that desorption is a thermal process at low excitation densities corresponding to low energy transfer rates. Ablation occurs at high excitation densities for which the vibrational to translational energy transfer rates produce a pressure pulse causing rapid expansion. The ejections in the two regimes were shown to exhibit distinctly different properties. At high excitation density the ejection in the early stage is dominated by forward directed, "hot" clusters that evaporate, so the ablation process may be thought of as forming a nanospray.

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