The rotational reflection principle in photodissociation dynamics

Abstract

Rotationally inelastic effects in the direct dissociation of a triatomic molecule are investigated. The calculations are performed on three levels of accuracy: the exact close-coupling method, the energy sudden approximation and very simple classical trajectory calculations. Rotational state distributions are usually smooth and highly inverted. They are explained as a one-to-one mapping of the ground-state wavefunction onto the quantum-number axis mediated by the so-called classical excitation function obtained by exact trajectory calculations on the excited-state potential-energy surface. We call this effect the rotational reflection principle. It should be applicable whenever many states are accessible provided that the rotational–translational coupling is strong enough to excite them. Under certain circumstances rotational rainbows due to the maximum of the excitation function become prominent. The rotational reflection principle establishes a unique relation between the angular part of the ground-state wavefunction, the anisotropy of the excited-state potential and the final rotational distribution. Many recent experimental distributions are qualitatively similar to those predicted by the rotational reflection principle. In two cases, ClCN and H₂CO, quantitative calculations on ab initio potentials are performed, and good agreement with experiment is found.