Mathematics – Probability
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........16d&link_type=abstract
Thesis (PH.D.)--THE OHIO STATE UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 56-12, Section: B, page: 6
Mathematics
Probability
Carbon Monoxide
Scientific paper
Both classical and mixed quantum-classical approaches have been used to study the non-thermal desorption of CO from a variety of model surfaces to which it is weakly adsorbed. In addition to three degrees of freedom for the CO adsorbate (bond stretching, physisorption, libration) which are treated quantum mechanically in the mixed method, a significant number of lattice degrees of freedom have been included using the generalized Langevin approximation. In the mixed method, two sets of equations for the quantum and classical subsystems (coupled via the Ehrenfest theorem) are solved self-consistently using the discrete variable representation method for the propagation of the quantum wave function. Non-thermal amounts of energy have been put into both the CO stretching and librational modes at t = 0. We find that for initial values of the stretching quantum number vstr = 0 -4 desorption does not take place at all within the simulation time unless there is also significant librational excitation. The detailed mechanism by which librational energy causes desorption is discussed. The role of the surface is also explored; we find that the probability of desorption is a non-monotonic function of the Debye frequency of the solid in the range 28-5000 cm^{-1}, and is larger for "non-rigid" lattices with low Debye frequencies in both of the methods. The classical results are explained in terms of resonances between low frequency libration and physisorption modes and high frequency phonon modes. For the combined method, two different mechanisms for desorption (due to lattice effects and due to symmetry properties of wave function) have been found and analyzed in detail. A comparative analysis of the two methods is presented and the limitations of the mixed scheme are discussed. An indirect mechanism for populating rapidly desorbing, highly excited levels of surface CO, based on intermolecular V to V,R energy exchange between the CO and vibrationally excited surface hydroxyl radicals has been proposed. The rate of the zero-phonon, V to V,R near resonant energy exchange between the OH(v = 1) and CO(v = 1, J = 7) molecules on ice, induced by a dipole-dipole interaction between the molecules, has been calculated using Fermi's golden rule. Average values of the reduced rate coefficient for this process, which leads to the population of a highly excited librational level of the CO adsorbate, have been found to be 5 times 1010 - 1 times 10 11s^{-1} depending on the orientation of the two molecules. The non-resonant zero-phonon and phonon-assisted analogous energy exchange processes on silica have been found to be significantly slower.
Dzegilenko Fedor N.
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