Metastable rovibrational f -symmetry levels of the I 1 Π g state of H2, HD and D2: experiment and theory

Abstract

We present accurate experimental measurements of the lifetimes and term values of the metastable rovibrational f-symmetry levels of the I 1Πg state of HD. These levels lie above the second dissociation limit of hydrogen. The lifetimes were obtained directly from the observation of the time-dependent decay of the fluorescence. These levels decay by direct fluorescence and by predissociation after tunnelling through the potential energy barrier of the I 1Πg state. We present ab initio results for both of these decay processes for H2, HD and D2. The theoretical calculations provide an explanation of the rotational dependence of the observed lifetimes for HD, the lack of rotational dependence for D2, and the approximately equal lifetimes for N = 1 for HD and D2. The calculated lifetimes for the metastable levels of H2 explain why no experimental data is yet available for the parent isotopomer. However, the use of recently updated Hönl–London factors with published ab initio electronic transition moments reveals a larger than expected discrepancy with experiment for those levels of HD and D2 for which emission is the dominant decay mechanism.