On isotope effects in the predissociations of HD

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Abstract
A discussion is given of the couplings which may give rise to an isotope effect in predissociation of diatomics with nuclei of equal charges but different masses. In the case of HD the coupled equations are numerically solved for model processes using published potentials and couplings from ab initio calculations. The results show that (i) isotope effects are due only to the second-order dynamic coupling between Born-Oppenheimer states and not to first-order radial coupling, (ii) these effects are controlled by the asymptotic form of the decrease of the gerade-ungerade splitting at the internuclear distance where it equals the splitting between the isotopic exit channels, (iii) in particular, strong isotope effects may occur on paths diabatically correlated with atomic states optically connected to each other, whereas no strong isotope effect can occur on the path diabatically correlated with the ion pair, (iv) the relevance of Demkov's model to this problem, as developed by van Asselt, Maas and Los, is confirmed. From these results a discussion is given of the predissociation mechanisms of some high Rydberg states of the hydrogen molecule