Atom-Diatom Scattering. From Potential Energy Surfaces to Rate Constants

Abstract: This thesis is about the theoretical study of collisions between atoms and diatomic molecules. It might seem like a trivial problem, but in reality it is a highly complex process. Despite, or because of, their apparent simplicity, these processes are of importance for a broad area of science. The applications stretch from the study of fundamental reactions at ultracold temperatures, through the far reaches of space, to the chemistry in the Earth’s atmosphere, and combustion at extreme temperatures. High-level ab initio methods (CASSCF/CASPT2) have been used to calculate potential energy surfaces in the linear 2? and 4??, and the non-linear 4A'' symmetry for the CNO system. The coupling between the collinear 2? and 4?? surfaces has been calculated with the CASSCF/RASSI method. The collinear surfaces have been interpolated using a Generalized Discrete Variable Representation method to produce potential energy surfaces as functions of the nuclear coordinates, while the global 4A'' surface has been fitted to analytical functions using many-body expansion. Time-dependent wave packet and quasiclassical trajectory calculations are presented for the O + CN reaction on the two collinear surfaces, both with and without coupling between them. Quasiclassical trajectory calculations are presented for the C + NO reaction on the 4A'' potential energy surface. The results from the calculations for the C + NO reaction are combined with previously presented results for the 2A' and 2A'' surfaces, and compared with experiments. It is seen that the new 4A'' surface significantly improves the agreement with experiments. This is the first published study of the 4A'' surface for the CNO system. Time-independent quantum mechanical methods have been used to study the inelastic collision of various atomic and molecular systems, at temperatures ranging from the ultracold to 10 000 K. Rate constants for the fine-structure excitation in the C + H and O + H collisions are presented. These collisions are of interest in astrophysics, as the results can be used to study the chemical evolution in interstellar clouds. The fundamental and very important spin-orbit relaxation of F(2P1/2) and Cl(2P1/2) atoms in a gas of H2 is investigated, and cross sections and rate constants are presented. Scattering processes at low temperatures in external electric and magnetic fields are also studied. It is shown that the spin-orbit relaxation of polar molecules in a buffer gas of He can be effectively controlled by the field strengths and the angle between the fields, at temperatures easily reached in the lab. This is the first report of the effect of crossed electric and magnetic fields on scattering processes.