Statistics – Computation
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........59g&link_type=abstract
Thesis (PH.D.)--DUKE UNIVERSITY, 1993.Source: Dissertation Abstracts International, Volume: 54-12, Section: B, page: 6223.
Statistics
Computation
2
Scientific paper
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 ~1 times 10^4 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.
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