Abstract
Oxygen molecules in the atmosphere protect us from harmful solar ultra-violet radiation through several mechanisms. Absorption of light (200-240 nm) by oxygen is known as the Herzberg continuum, associated with the excitation of ground state oxygen to the three states A 3\Sigma_u +, c 1\Sigma_u -, A' 3\Delta_u. The excitation of O_2 in the Herzberg continuum, which is electric dipole forbidden, is a complicated process, since many electronically excited states and spin-orbit and orbit-rotation couplings amongst them are involved. The subsequent photodissociation process that determines the fine structure distribution and polarization of the atomic fragments also involves many states and couplings, in particular for large internuclear distances. In 1998 a photoabsorption model was constructed by Buijsse et al. that can be used to calculate the photoabsorption cross sections in the Herzberg continuum as a function of wavelength. This model was validated by advanced experiments in which so called fine structure resolved anisotropy parameters were determined. The model used by Buijsse et al. to describe the experiments was based on a simplified description of the mechanisms. This thesis provides a comprehensive theoretical description and high level ab initio and dynamical calculations of the relevant mechanisms. In Chapter 2 we present the calculated potential energy curves for a set of O_2 excited states, spin-orbit couplings, and the radial derivative couplings. In Chapter 3 these potentials and couplings are used in a semiclassical calculation of the photodissociation process. In Chapter 4 we employ our potentials and spin-orbit couplings to compute perturbations in the observed Herzberg rotational-vibrational-electronic levels just below the dissociation limit, that are caused by bound states in the 3\Pi_u state. In Chapter 5 we present a fully ab initio study of the excitation mechanism. We present calculations of ten electronically excited intermediate states and couplings that give intensity to the Herzberg transitions
