Molecular dynamics simulations of energy transfer and laser ablation in organic solids

Laser ablation is used in various applications such as medical surgery and Matrix Assisted Laser Desorption Ionization (MALDI). In spite of the success of laser ablation in various applications, the manifold facets of the ablation process are still not well understood. In this thesis, molecular dynamics simulations on the atomic and molecular scale are used to study the rates and mechanisms of energy transfer and laser ablation in organic solids.;Atomic level molecular dynamics simulations were performed using the MM3 potential to study the intramolecular and intermolecular energy transfer processes in benzene. The simulations show that the intramolecular energy transfer process is ultrafast, i.e., 0.5 ps. Classical dynamics simulations are able to reproduce the deviations from the Fermi Golden Rule that are observed experimentally at high vibrational excitations.;Intermolecular energy transfer processes were studied in crystalline benzene. The rate of intermolecular vibrational energy transfer in benzene crystal was found to be ;Using the information from the energy transfer processes the breathing sphere model was developed for molecular dynamics simulations of laser desorption and 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. An apparent threshold fluence has been found to separate two mechanisms for the ejection of molecules--desorption 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 leads to the strongly forwarded emission of ablated material. Big molecular clusters are found to constitute a significant part of the ejected plume at fluences right above the ablation threshold. At fluences below the threshold fluence there is a solid to liquid phase transition leading to desorption of molecules with similar average axial kinetic energies.