Anomalous rotational-state distribution of NO A state in UV photodissociation of rare gas–NO van der Waals complexes. Rotational rainbow effect

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Abstract
Rotational distribution of NO in the Rydberg A state produced in photodissociation of the Ar–NO and Ne–NO van der Waals complexes have been measured for the first time by using a (1+1) resonant ionization technique in the region 44 350–44 800 cm−1. Two eminent maxima have been observed in the distributions at all the laser energies studied. Dependences of these maxima on the excess energy and the reduced mass have been found to be the same as those of the rotational rainbow peaks in rotationally inelastic scattering. Theoretical formulation is given here for the photodissociation of a triatomic molecule. With a half‐collision approximation and a certain assumption about the Franck–Condon factor, the photodissociation cross section has been shown to be proportional to a partial cross section for rotationally inelastic atom–diatom collision. The partial cross sections are calculated by the quantum mechanical close‐coupling method with the use of a model potential which mimics the potential for the relevant ArNO system. These calculations strongly support the experimentally observed anomalous rotational‐state distribution is a manifestation of the rotational rainbow effect. The upper limits of the dissociation energies of the van der Waals complexes have also been estimated from the highest accessible rotational levels of the photodissociation products.