Quantum dynamics of molecular photodesorption from metal surfaces

Abstract

We develop a theory of desorption of molecules from metal surfaces following absorption of visible or UV light pulses, based on the separation of the system into primary and secondary regions. Our approach introduces the Liouville–von Neumann equation for the density operator and derives from it the effective equations for the two coupled regions, assuming a self-consistent factorization of the density operator. The secondary region is described as a stochastic medium providing energy dissipation during the desorption. Direct and indirect desorption mechanisms are considered, with the latter derived from a local electromagnetic field which excites the metal substrate electron–hole excitations following which the adsorbate bond is broken. A model is introduced and solved for CO adsorbed on Cu(001). Results for the survival probability of adsorbed CO following absorption of a light pulse suggest, by comparison with experiment, that an enhanced transition dipole must be formed in the primary region.