This dissertation focuses on energy transfer (rotational, vibrational, and electronic) of CH₂ in its ground and first excited electronic states (X3B₁ and a1A₁), by collisions with the helium atom.

Initially we investigate energy transfer within the two electronic states separately. We carry out ab initio calculations to determine the potential energy surfaces for the interaction of He with CH₂ in these two states. The PES for He-CH₂(a) is more anisotropic than for He&mdashCH₂(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&mdashCH₂(X) we determined the dependence of the PES on the CH₂ 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 CH₂ 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.