Vibrational predissociation of H2-, D2-, and HD-Ar van der Waals molecules

Accurate close-coupling calculations are performed for vibrationally predissociating states of H2-Ar, D2-Ar, and HD-Ar, using the best potential energy surface available. All the states examined have very small widths (GAMMA << 10-6 cm-1, corresponding to lifetimes > 20 μs). There is a pronounced tendency for predissociation to yield rotationally hot diatomic molecules, even for the H2-Ar and D2-Ar complexes where the present potential has no anisotropic terms of higher order than P2(cos theta). This near-resonant effect is particularly strong for HD-Ar, where all Legendre terms are present in the potential; in this case, about 50% of the HD products are formed in the highest two accessible rotational levels. There is some evidence for a rotational rainbow effect in the product rotational state distributions. Perturbation theory calculations which attempt to reproduce the accurate calculations are also reported. They successfully model the qualitative features of the close-coupling results, but are not quantitatively accurate even for these weakly coupled systems. It appears that this inadequacy is due to the need for a very accurate representation of the bound state wave function and to the neglect of important couplings between the different open channels. This conclusion is supported by the observation that very large basis sets are required to obtain convergence of the close-coupling calculations. 4 figures, 7 tables