Mode-specific infrared photodissociation of nitric oxide dimers: High-resolution infrared spectroscopy of (14NO)2 and (15NO)2

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Abstract
The high‐resolution infrared absorption spectra of the symmetric (ν1) and the antisymmetric NO stretching (ν4) bands of nitric oxide dimer (NO)2 have been measured for 14NO and 15NO in supersonic free jets. The ν1 and ν4 bands exhibit a dramatic difference in linewidth: approximately 200 MHz [full width at half‐maximum (FWHM)] for the ν1 band and approximately 5 GHz (FWHM) for the ν4 band. The predissociation lifetimes deduced from the linewidths are in excellent agreement with those reported in the recent time‐resolved measurement for 14NO [Casassa e t a l., J. Chem. Phys. 8 9, 1966 (1988)]. There is no systematic dependence of the linewidth on the rotational states of (NO)2. Isotope substitution does not influence the linewidths significantly. However, the ν4band structure of (15NO)2 is very different from that of (14NO)2, a difference that may be explained by a perturbation from a low‐lying singlet vibronic state. All of the experimental results obtained to date may be accounted for if it is assumed that the predissociation of (NO)2 is enhanced by an electronically nonadiabatic transition to a repulsive triplet surface. Vibrational potential coupling between the NO stretching and intermolecular modes, particularly an in‐plane NO bending mode, appears to play a key role in the mode specificity.