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Photoinduced quantum dynamics in molecules and at adsorbates

A problem of long-standing scientific interest is the interaction of electromagnetic fields with atomic and molecular species both in isolation and within a medium, and the ensuing spectroscopy and chemical dynamics. This paper presents an overview of theoretical and computational methods, both fully quantum mechanical or semi-classical in nature, developed over many years at the Quantum Theory Project of the University of Florida for dealing with photo-induced processes such as molecular dissociation or desorption of adsorbates from metal surfaces. In the former case, photoinduced transition rates are computed from transition operators via eikonal functions, or more generally with a time correlation function in which the radiation and material degrees of freedom are treated separately. Application to the photodissociation of methyl iodide is outlined. For molecular photodesorption, the system is partitioned into two self-consistently coupled regions, and a density matrix formalism in Liouville space is employed to study its time evolution in the general case when the substrate is also photoexcited. Pulsed laser irradiation of the substrate, described by optical Bloch and kinetic rate equations, results in electronic to vibrational energy transfer from metal to adsorbate and subsequent photodesorption. The theory has been applied to the CO/Cu(001) system to calculate desoprtion yields. Also treated is the optical control of yields via chirping effects, which has been predicted to both suppress or enhance the amount of CO leaving the surface.