The rotational reflection principle in the direct photodissociation of triatomic molecules. Close-coupling and classical calculations

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Abstract
We investigate rotational state distributions following the direct photodissociation of triatomic model systems. In view of several recent measurements cases of very high rotational excitation are studied. The dissociation cross sections are calculated exactly by the quantal close-coupling method. All distributions are smooth and highly inverted depending sensitively on the anisotropy of the dissociative potential energy surface. They are explained as a mapping of the bound state wave function onto the quantum number axis. This mapping is mediated by the so-called classical excitation function which is determined by running classical trajectories on the potential energy surface within the dissociative state. We call this effect rotational reflection principle. It establishes a rather direct relation between the bound state wave function of the parent molecule, the anisotropy of the dissociative potential, and the final rotational state distribution. The classical calculations agree qualitatively and in most cases even quantitatively very well with the exact quantal results. Differences are most pronounced in the region of a rotational rainbow where the classical cross section is singular. Because of the special choice of initial conditions only very few trajectories are needed to get converged results.