ELECTRONICALLY DIABATIC QUANTUM DYNAMICS OF MOLECULAR DESORPTION

The photodesorption of a diatomic from a metal surface, following abso rption of visible or UV light, involves electronic transitions of the desorbing species coupled to the lattice vibrations and electron-hole excitations of the substrate. We present a general treatment of these phenomena, based on the Liouville-Von Neuman equation for the density operator, and a stochastic theory of localized perturbations in an ext ended system. The Hamiltonian of the extended molecular system is divi ded into a term for the localized primary degrees of freedom (DFs) aff ected by the desorption, coupled to secondary DFs that acts as a time- evolving bath. A self-consistent field treatment gives an effective (n on-Hermitian) Hamiltonian for the primary DFs that accounts for energy fluctuation and dissipation in terms of the properties of adsorbate a nd substrate. A diabatic electronic representation is used to eliminat e momentum couplings between adsorbate electronic states. The bath dyn amics is studied for lattice vibrations and for electronic excitations . Electron-hole excitations of the substrate are considered for intrab and and interband transitions. The assumption of Brownian motion leads to expressions for the dissipative potentials in terms of the time-co rrelation functions of lattice displacements and of electron density f luctuations. The dissipation depends on time, allowing for time-depend ent substrate temperatures and generalizing the Langevin description. Dissipation contributes to the time evolution of both ground and excit ed electronic states of the desorbing species. The model is discussed for the special case of Ni(001)(ads)CO. (C) 1995 American Institute of Physics.