State resolved studies of photochemical dynamics at surfaces

Abstract

Photochemical surface processes are initiated by the absorption of photons by an adsorbate or the substrate, and followed by various energy transfer and relaxation processes that lead to nuclear motion along the reaction coordinates. A powerful way to gain insight into the molecular dynamics of such processes involves the determination of final state distributions in the translational and internal degrees of freedom of desorbed product molecules. These distributions include the angular distribution, the translational, vibrational, and rotational energy distributions, the spatial alignment of angular momentum vectors, and populations of spin-orbit and lambda doublet states. Additional information is contained in correlations between two or more degrees of freedom, e.g., the velocity distribution as a function of desorption angle or internal state, or correlations between spin-orbit and rotational state populations. In this Report we review state resolved studies of surface photochemistry (hν ⩽ 6.4 eV) for which the final state distributions have contributed to our understanding of the microscopic dynamics, focusing on recent progress in our ability to explain the observed distributions with simple dynamical models. After briefly reviewing basic photochemical excitation and reaction mechanisms at surfaces, we discuss experimental techniques, including time-of-flight measurement by mass spectrometer and laser spectroscopic techniques. We then review vibrational distributions of photochemically desorbed molecules, interpreting them in terms of a simple quantum dynamics model. A section on translational distributions discusses dynamical models explaining observed velocity distributions of photochemically desorbed species for both weakly and strongly quenched systems. Next we review rotational distributions of photochemically desorbed molecules, and present a dynamical model that can explain many of the observed distributions, as well as the ubiquitous positive correlations between the rotational and translational degrees of freedom. After discussing recent measurements of the spatial alignment of angular momentum vectors of photodesorbed molecules, we review models explaining spin-orbit state propensities and spin-orbit-rotational correlations for photodesorption and surface photodissociation systems.