The classical and quantum dynamics of desorption and vibrational energy relaxation

Interstellar molecular clouds are composed of a gas phase consisting of atomic and molecular species and a condensed phase in the form of dust grains. The importance of non-thermal desorption mechanisms to cloud chemistry is currently not well understood. In this thesis, desorption from dust grain surfaces via the excitation of an internal vibrational mode of a physisorbed molecule has been considered.; Desorption probabilities and rates of vibrational energy relaxation have been computed using two methods. In the first method, classical trajectories were computed using the generalized Langevin equation; the adsorbate translational and internal vibrational modes and the lattice (dust grain) vibrations were treated. In the second, the adsorbate coordinates have been treated using quantum mechanics; the lattice vibrations have been handled classically.; A thorough classical trajectory study has been performed for a model of CO adsorbed on a dust grain composed of silicon dioxide. Desorption from the first excited intramolecular vibrational state has not been observed. Quasi-bound desorption does occur in the rigid lattice approximation for large vibrational energies. In non-rigid lattice computations, however, no quasi-bound desorption has been observed. The desorption that does occur involves trajectories with a single adsorbate (vertical) translational turning point; these trajectories are probably not relevant to the chemistry of interstellar clouds. A number of computations have been performed for CO adsorbed on a lattice with a vibrational spectrum that is qualitatively similar to that of a grain mantle composed of non-polar ices. For very large vibrational energies the non-rigid lattice desorption yields are appreciable. The relevance of these results to the gas-grain chemistry of interstellar clouds has, however, not been established.; Several computations have been performed using the mixed classical-quantum method. The desorption yields are qualitatively in agreement with the corresponding classical trajectory results. In particular, the desorption lifetime is estimated to be greater than {dollar}sim{dollar}1 {dollar}times{dollar} 10{dollar}sp4{dollar} picoseconds for vibrational quantum numbers less than 35, assuming that the lattice is rigid. It has also been shown that the classical-quantum method treats the interaction between the classical and quantum subsystems unphysically.