State-to-state vibrational predissociation dynamics and spectroscopy of HeCl2: Experiment and theory

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Abstract
The structure and vibrational predissociation dynamics of HeCl2 are studied by pump–probe spectroscopy and by three‐dimensional quantum mechanical calculations. Parity selected excitation spectroscopy is used to confirm the essential features of the previous analysis of the HeCl2 B←X laser excited fluorescence spectra. Product vibrational and rotational state distributions are measured for the v’=6, 8, 12, 20, and 24 levels of HeCl2 in the B state. For the v’=6 and 8 levels the dependence of the product state distribution on the initially excited rotational state is also measured. Although the dissociation dynamics are dominated by Δv=−1, V→T energy transfer, several interesting effects are revealed by monitoring the product rotational degrees of freedom. Due to the symmetry of the HeCl2 potential, the parity of the initially excited HeCl2 rotational state is conserved during the dissociation dynamics. Even when a single initial rotational state is excited, the observed product rotational state distribution is bimodal. The product rotational distribution is nearly independent of the amount of kinetic energy released to the product degrees of freedom. Three‐dimensional quantum mechanical calculations using a simple potential energy surface are remarkably successful at reproducing the details of the experimental measurements. Only five parameters of the potential were adjusted to calculate the excitation spectrum, the vibrational predissociation product state distributions, and the lifetimes of the excited states. Analysis of the dissociation mechanism in terms of simple models, however, is not straightforward. In particular, the impulsive, quasiclassical half‐collision model is not compatible with the observed independence of the product rotational state distribution from the amount of kinetic energy which is released. The close agreement between state‐to‐state experiment and quantum theory on the HeCl2 dynamics shows that the shortcomings of more approximate theories are fundamental and cannot be attributed to lack of knowledge of the true potential energy surface.