Molecular dynamics model for laser ablation and desorption of organic solids

A breathing sphere model is developed for molecular dynamics simulations of laser ablation and desorption of organic solids. An approximate representation of the internal molecular motion permits a significant expansion of the time and length scales of the model and still allows one to reproduce a realistic rate of the vibrational relaxation of excited molecules. We find that the model provides a plausible description of the ablation of molecular films and matrix-assisted laser desorption. An apparent threshold fluence has been found to separate two mechanisms for the ejection of molecules: surface vaporization at low laser fluences and collective ejection or ablation at high fluences. Above threshold the laser-induced high pressure and the explosive homogeneous phase transition lead to the strongly forwarded emission of ablated material and high, from 500 up to 1500 m/s, maximum velocities of the ejected plume expansion. Large analyte molecules get axial acceleration from an expanding plume and move along with the matrix molecules at nearly the same velocities. Big molecular clusters are found to constitute a significant part of the ejected plume at fluences right above the ablation threshold. The processes in the plume are found to have a strong influence on the final velocities of ejected molecules and molecular clusters.