|dc.description.abstract||This dissertation focuses on energy transfer (rotational, vibrational, and electronic) of CH<sub>2</sub> in its ground and first excited electronic states (X<super>3</super>B<sub>1</sub> and a<super>1</super>A<sub>1</sub>), by collisions with the helium atom.
Initially we investigate energy transfer within the two electronic states separately. We carry out <italic>ab initio</italic> calculations to determine the potential energy surfaces for the interaction of He with CH<sub>2</sub> in these two states. The PES for He-CH<sub>2</sub>(a) is more anisotropic than for He&mdashCH<sub>2</sub>(X). In the former case we perform quantum scattering calculations and report state-to-state and overall removal cross sections, from which we compute room temperature rate constants.
For He&mdashCH<sub>2</sub>(X\) we determined the dependence of the PES on the CH<sub>2</sub> bending degree of freedom. By averaging over the bending vibrational wave functions, we were able to investigate collisional relaxation of both rotation and the molecular bending. The PES of the X state is less anisotropic than that of the a state, resulting in a less efficient relaxation process. Vibrational relaxation is very inefficient, with cross sections less than 1% of those for rotational relaxation.
By taking into account the weak spin-orbit coupling between the a and X states, we explore collision-induced electronically inelastic processes. We invoke, the mixed-state model, in which transitions are due entirely to the mixing of nearly-degenerate pairs of rotational levels. We compare the computed removal rate constants with experimental results by Hall and Sears at Brookhaven.
Finally, we simulate the time evolution of the singlet-triplet relaxation of CH<sub>2</sub> by solving the relaxation master equation. The simulation shows that relaxation occurs in three stages: immediate re-distribution between the two mixed states, fast rotational relaxation within the a state and a much slower relaxation within the X state. Eventually, most of the population relaxes to the X state.||en_US