Photodissociation and radiative stabilization of HeH^+ in its a and b ^3Sigma^+ states (original) (raw)

Proceedings of The International Astronomical Union, 2011

Abstract

According to models, the HeH^+ cation is the first molecular species to have formed in the early universe, by radiative association of H^+ with He or H with He^+. Although it is predicted to be abundant in He-rich astrophysical environments, such as planetary nebula NGC7027, it has so far eluded extra-terrestrial observation. It is proposed here that the HeH^+ ion could be detected in its a ^3Σ^+ metastable state, as it has a long lifetime (< 100 s). The first superior electronic state of the same symmetry, the b ^3Σ^+ state, has a much shorter lifetime (≈ 10-8 s) and will thus populate the a state radiatively. We investigate the formation of HeH^+ in the a and b ^3Σ^+ states by radiative stabilization and their destruction by photodissociation using time-dependent quantum dynamics. The partial photodissociation cross-sections from all their vibrational levels towards the ten ^3Σ^+ and the six ^3Π superior states are calculated, from which the corresponding radiative stabilization cross-sections are then inferred. The rate constant for both processes is estimated in the range of temperature and of radiative energy distribution found in planetary nebulae. It is shown that the vibrational dependence of the radiative stabilization cross-section, usually neglected in the estimation of the rate constant, has a significant impact at low temperature.

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