Theoretical study of the NO γ system

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Abstract
The potential curves for the X 2Π and A 2Σ+ states of NO have been determined using large Gaussian basis sets and accounting for electron correlation with the MCPF and CASSCF/multireference CI methods. The A 2Σ–X2Π electronic transition moment (γ system) is found to increase monotonically with decreasing internuclear distance. It differs significantly from the functional form deduced empirically from either branching ratio or lifetime measurements. The increase in the moment as r decreases is correlated with the increasing degree of diffuse character in the X 2Π state. Inclusion of 2s correlation is found to have a relatively small effect on the functional form of the moment once the valence correlation is accurately determined. However, the NO γ system transition moment is found to be sensitive to the choice of the molecular orbital basis. The X 2Π and A 2Σ+ dipole moment functions are studied to provide an additional assessment of the quality of the wave functions. The X 2Π vibrationally averaged dipole moments and the (1–0) and (2–0) vibration–rotation band intensities are in excellent agreement with the experimental values.