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 1 Å g 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 1 Å g state. We present ab initio results for both of these decay processes for H 2 , HD and D 2 . The theoretical calculations provide an explanation of the rotational dependence of the observed lifetimes for HD, the lack of rotational dependence for D 2 , and the approximately equal lifetimes for N ¼ 1 for HD and D 2 . The calculated lifetimes for the metastable levels of H 2 explain why no experimental data is yet available for the parent isotopomer. However, the use of recently updated Ho¨nl-London factors with published ab initio electronic transition moments reveals a larger than expected discrepancy with experiment for those levels of HD and D 2 for which emission is the dominant decay mechanism.

Spectral identification of diffuse resonances in H2 above the n = 2 dissociation limit

“…In the work of Ross et al 18 predictions were made on the lifetimes of the I 1 − g , v = 4 levels, which were indicated as metastable. In a more sophisticated treatment, including the emission rate and an improved calculation of the oscillation frequency factor f v accounting for the "softness" of the potential energy barrier with a quantum correction factor, 38 they find values (also listed in Table I) and is, therefore, assigned to a state.…”

“…18 The level structure of singlet-g states was unraveled based on nonadiabatic ab initio calculations in the comprehensive work by Yu and Dressler. 19 Ross and Jungen succeeded in providing assignments of the bound gerade levels below the n = 2 limit via multichannel quantum defect theory (MQDT).…”

Born–Oppenheimer breakdown and non-adiabatic lifetimes of rovibrational levels of D2 lying near the n=2 dissociation limit: Experiment and theory

“…For example this was the case in the work reported in Refs. [2][3][4]. However, although the adiabatic approach accounts for the electronically-diagonal effects of T vib in (4), it still neglects the effects of T vib between different electronic states.…”

“…[2] for the H 1 R g + state, in Ref. [3] for the metastable levels of the inner well I 1 P g state, and in Ref. [4] for the long-range levels of the outer well I 0 1 P g state.…”

“…In their 2002 review article on the lifetimes of rovibronic states of the hydrogen molecule, Astashkevich and Lavrov pointed out that, other than for H 2 , lifetime data for hydrogen molecule isotopologues was very fragmentary [1]. In light of these two observations we have been investigating lifetimes of excited rovibronic levels of H 2 , HD, and D 2 lying near the n = 2 dissociation limit [2][3][4]. In this experimental-theoretical collaboration we have concentrated on those situations where the lifetimes vary dramatically with isotopomer, vibrational level, or rotational excitation.…”