Quantum State Resolved Studies of Photodesorption Dynamics

Photodesorption is a fundamental photochemical surface process, in which absorption of light by adsorbed molecules or by the substrate causes ejection of molecules into the gas phase. The goal of the research presented in this dissertation was to investigate the elementary steps of energy transfer and relaxation involved in photodesorption. It was demonstrated that measuring internal and translational state distributions of photodesorbed molecules is a powerful way to gain insight into the physical mechanisms and dynamics of photochemical surface processes. The dynamics of ultraviolet laser (193 and 308 nm) induced desorption of CO from thin epitaxial NiO(111) films, and from Si(100)-c(2 times 4) was studied by detecting desorbed CO molecules in specific quantum states using laser induced fluorescence (LIF) spectroscopy. This technique allowed measuring the final state distributions in desorption angle and velocity, vibrational and rotational quantum states, and the angular momentum alignment, as well as correlations between two or more degrees of freedom. The desorbed molecules have nonthermal amounts of energy in rotational, vibrational, and translational motion. CO photodesorbed from oxidized Ni(111) is characterized by a Maxwell-Boltzmann velocity distribution, and a rotational distribution which is described as a superposition of two Boltzmann distributions. These distributions are the same for the two desorption wavelengths studied, indicating a substrate mediated excitation mechanism. The final state distribution of CO photodesorbed from Si(100) consists of two distinct components. One is rotationally hot but translationally relatively slow and exhibits a preference for cartwheeling alignment. The other component is rotationally quite cold but translationally fast, with a preference for helicoptering alignment. The observed distributions, as well as those previously observed for other photodesorption systems are discussed in terms of a simple dynamical model which circumvents our ignorance of excited-state potential energy surfaces by exploiting the fast electronic relaxation rates encountered at metal and semiconductor surfaces. The model explains rotational Boltzmann distributions, Maxwell-Boltzmann velocity distributions, and rotational-translational correlations characteristic of photodesorbed molecules, as well as spin -orbit state populations of photodesorbed NO. With certain assumptions about adsorption geometries, it can quantitatively explain the measured angular momentum alignment.