Probing the Dynamics of Gas-Surface Interactions with Rotationally Inelastic Scattering

The goal of this research project was to obtain information about the dynamics of gas-surface interactions. In particular, this research examines the effects of molecular orientation geometry on the interaction potential between molecules and surfaces using rotationally inelastic scattering. The inelastic scattering experiments were performed by scattering a rotationally cold, monoenergetic beam of molecules off a clean single crystal metal surface held in ultra-high vacuum. The scattered molecules were subsequently detected using rotational and vibrational state resolved and polarization sensitive resonance enhanced multiphoton ionization (REMPI) spectroscopy. The primary advantage of these experiments is the specificity of the information obtained with our spectroscopic detection and the well defined initial conditions provided by the molecular beam and surface techniques. The detailed information obtained from these experiments have helped us to intuitively understand chemical dynamics on surfaces and enabled us to rigorously compare our observations with theoretical calculations. For example, when an ellipsoid scatters off a hard, flat, frictionless surface, the amount of rotational excitation depends upon the initial orientation of that ellipsoid with respect to the surface. Experimentally, we can infer the initial molecular orientation of molecules in a given system by evaluating the amount of rotational excitation in the scattered distribution. Some systems that were examined include CO scattered from Ag(111), N _2 scattered from Pt(110), Pt(111), Ag(111), and W(110), and N_2 and H_2 desorbed from W(110). In addition to the dynamics experiments, we also performed a detailed study of the the REMPI spectroscopy of N_2, which proved invaluable in the dynamics studies involving N_2, and we examined the state-resolved photodissociation of N_2 O.