Theoretical study of competing photoionization and photodissociation processes in the NO molecule

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Abstract
The multichannel quantum defect theory of dissociative recombination is reviewed and adapted to treat simultaneous vibrational preionization and electronic predissociation in NO. The photoionization, photodissociation, and photoabsorption cross sections of NO corresponding to 2Π excited states are computed in the range from the N(2D)+O(3P) dissociation limit up beyond the second vibrational level of NO+(X 1Σ+). The calculations are based on the 2Π Rydberg and valence state potential curves and Rydberg-valence state interaction energies extracted by Gallusser and Dressler from the high-resolution spectroscopic data of Miescher and co-workers. These data pertain to the discrete range and furnish precise information on numerous spectral perturbations involving levels of the npπ Rydberg series and the B and L valence states. The calculated cross section profiles reproduce with reasonable accuracy the observed absorption and photoionization curves, and in particular the widths of the Rydberg resonances. The coexistence of Δv=1 and Δv≳1 preionization mechanisms is demonstrated. While the Δv=1 process is related to the R dependence of the quantum defect, the Δv≳1 preionization is shown to be induced by the strong electronic Rydberg-valence state coupling which primarily (and simultaneously) is responsible for predissociation. This result is generalized to the competition between any different decay mechanisms affecting a discrete excited level, and several limiting cases of competition are characterized. The relative contributions of different partial waves lλ to the photoionization spectrum of NO are estimated on the basis of the medium-resolution Rydberg absorption spectrum.