Rotational energy transfer within the Ã2A1state of NH2: absolute rate coefficients and the influence of spin exchange in collisions with H atoms

Abstract
Experiments are described which concern rotationally inelastic events occurring within the (0, 9, 0) vibronic level of the Ã2A1state of NH2. Continuous-wave (c. w.) dye-laser excitation and dispersion of the result­ing fluorescence allows the observation of specific rotational state to state processes. It is demonstrated experimentally that hydrogen atoms are the dominant agent in causing these inelastic events. Absolute rate coeffi­cients for state to state transfer are quoted and are shown where relevant to agree with the principle of microscopic reversibility. Propensity rules are clearly evident, and it is found that transitions with ΔK= 0 and 2 occur with very similar rates. The rate data are interpreted in terms of the restricted distorted-wave Born model. This model is found to give a semi-quantitative fit to the experimental data. In order to explain anomalies in rates among levels of para symmetry a collisionally induced diabatic mechanism is proposed in which high levels of the ground X~2B1state of NH2are involved. The electronic spectrum of NH2possesses resolvable doublet structure. A series of experiments are described in which the relative populations within collisionally populated spin doublets were measured. These results are interpreted in terms of the theory set out in the preceding paper (Dixon & Field 1978) for encounters in which both collision partners are open-shell. We show that our findings point to a marked spin correlation in the interaction potential. The rate data allow us to draw conclusions regarding the anisotropy of the potential of interaction of H and NH2. The analysis of the spin branch­ing data indicates the strength of the anisotropy of the triplet and singlet surface to be approximately in the ratio 1:4. Such conclusions are found to be consistent with the physical picture that emerges from a qualitative knowledge of surfaces of the NH3system. The general relevance of our data to laser modelling and of spin exchange to interstellar maser action is noted.