| Summary: | This thesis describes studies of collisional energy transfer in two small, combustion relevant free radicals. Collision-induced electronic energy transfer (EET) between the B2r and A2 fj. states of the CH and CD radicals was investigated with the collision partners He, Ar, H2, N2, CO and C02 using a dispersed laser induced fluorescence technique. CH or CD radicals were prepared photolytically, and excited into a single rotational level in either of the B2::E-, v ==°or A211, v =1 levels. Wavelength dispersed, time-resolved emission was then recorded from the initial and product states. Microscopic rate constants for vibronically resolved transfer between the two electronic states were determined for each collision partner, as well as those for vibrational energy transfer in the A state and total removal to other, unobserved states. EET was demonstrated to be ubiquitous, occurring with all of the collision partners used in the study, with varying efficiencies. These relative efficiencies were found to correlate well with long range attractive forces exerted by the collider. No special enhanced efficiency was observed with those colliders chemically reactive towards CHID. Transfer from B2::E- to A2 1::,. was not greatly affected by the significant differences to the vibronic energy level structure -between CH and CD, demonstrating the inapplicability of empirical energy gap scaling laws to this process. The collisional evolution of alignment and orientation moments in the OH radical was investigated using polarisation spectroscopy (PS). One-Colour PS measurements were made using the first five P branch lines in the A2::E+ - x2n (0,0) band of OH, in the presence of fixed pressures of He, Ar or N2. The time delay between pump and probe laser pulses was varied to inspect the remaining alignment or orientation in the sample as a function of time after the pump pulse. The results of these experiments were analysed using a detailed theoretical treatment developed previously in the laboratory, producing rate constants and cross-sections for the collisional decay of alignment and orientation over the range of quantum states and collision partners studied. While some uncertainty exists in the numerical values of the results, due to the observation of an apparently pressure-independent depolarisation process, clear phenomenological distinctions were found between the collision partners He, Ar and N2, over the range of studied quantum states.
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