Molecular dynamics model for laser ablation of organic solids

A breathing sphere model is presented for molecular dynamics simulations of laser ablation 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. The model provides a realistic description of the ablation of molecular films and matrix-assisted laser desorption (MALDI). A well-defined threshold fluence has been found to separate two mechanisms for the ejection of molecules - surface vaporization for low laser fluences and collective ejection or ablation for high fluences. Above the threshold the laser induced high pressure and the explosive homogeneous phase transition leads 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 in MALDI 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 and angle distributions of ejected molecules and molecular clusters.