Scattering of State-Selected and Oriented Hydroxyl Radicals by Halogen Hydrides and Xenon

Abstract

The interaction of the OH radical with atoms and other molecules is relevant for many physical and chemical processes involved in atmospheric, combustion and interstellar chemistry. Various experimental and theoretical studies have revealed information concerning the interaction of the hydroxyl radical with rare gas atoms like He, Ar, Ne and molecules like H2, N2, CH4, CO and CO2. A remarkably good agreement between the theoretical and experimental results was obtained for the OH-Ar inelastic scattering, confirming thus the accuracy of the theoretical models. In this thesis crossed molecular beams experiments are presented in which the scattering of OH molecules by HCl, HBr, HI and Xe is studied. State-to-state cross-sections and steric asymmetries are measured which probe the complete Potential Energy Surfaces (PES) for these atom-diatom or diatom-diatom systems. The experimental techniques employed are: supersonic molecular beam expansions, electrical discharge, electrostatic fields to focus and select the OH molecules in a single internal energy state and to orient them with the H-side or the O-side towards the collision partner, laser induced fluorescence, resonance enhanced multiphoton ionization and velocity map imaging. The rotational inelastic cross sections obtained indicate similarities among the OH-HCl, OH-HBr and OH-HI scattering systems. Generally, the spin-orbit conserving transitions dominate, the -doublet transition exhibits the largest cross-section and the magnitude of the cross-sections decreases with rotational excitation. The measured steric asymmetries reflect the higher anisotropy of the OH-HCl interaction potential. Small steric asymmetries are measured for OH-HBr and OH-HI. A possible correlation between the magnitude and sign of the experimental steric asymmetries with the molecular configurations for the PES minima and the occurrence of the reactions is considered. In order to enrich the knowledge of the OH-rare gases interaction, state-to-state inelastic cross sections and steric asymmetries for the OH molecule scattered by Xe are presented. The experimental results are compared with the theoretical results and with the previously reported results for OH-He and OH-Ar scattering. The major differences observed are discussed aiming at the understanding of the contribution to the interaction of specific type of the intermolecular forces.