MIXED QUANTUM-CLASSICAL CALCULATIONS ON THE NONTHERMAL DESORPTION OF PHYSISORBED CO

A combined quantum-classical approach has been used to study the nonthermal desorption of CO from a variety of model surfaces to which it is weakly adsorbed. Three degrees of freedom associated with the CO adsorbate (bond stretching, physisorption, libration) are treated quantum mechanically, while classical mechanics is applied to the lattice degrees of freedom, which have been included using the generalized Langevin approximation. 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. Nonthermal amounts of energy have been put into both the CO stretching and physisorption-librational modes at time t=0. We find that for initial values of the stretching quantum number nr=0-4, desorption does not take place at all within 22.5 ps unless there is also significant librational excitation. The role of the surface is also explored; we find that the probability of desorption is a nonmonotonic function of the Debye frequency of the solid in the range 28-5000 cm -1, and is larger for "nonrigid" lattices with low Debye frequencies. Two different mechanisms for desorption (due to lattice effects and due to symmetry properties of the wave function) have been found and analyzed in detail. © 1995 American Institute of Physics.