First-principles calculations on the astrochemistry and spectroscopy of H +3

Abstract

The H⁺₃ molecular ion features prominently in interstellar medium (ISM) reaction schemes. It is the main protonating agent and provides a major channel for deuterium fractionation. Yet all attempts to detect H⁺₃ in the ISM by using infrared absorption techniques have so far failed.

We have performed a series of ro-vibrational calculations of spectroscopic accuracy using a high-quality potential-energy surface due to Meyer, Botschwina and Burton. With these we have built up an extensive data set of ro-vibrational energy levels and transition intensities. From the energy levels we have generated partition functions and hence temperature-dependent equilibrium constants for the fractionation reactions H⁺₃+ D → H₂D⁺+ H and H⁺₃+ HD → H₂D⁺+ H₂, as well as the H⁺₃ forming reaction H₂+ H⁺₂→ H⁺₃+ H. At the low temperatures typical of the ISM, the fraction of H₂D⁺ rises rapidly and our theoretical estimates are at their most reliable.

Emission spectra of H⁺₃ from both the ν₂ and 2ν₂ bands have been observed by several groups in the polar regions of Jupiter. These are now being used to both monitor and map the complicated auroral activity of Jupiter. Recently, we have also assigned v₂ emissions in the spectrum of supernova SN1987a and observed these emissions in Uranus. These successes suggest that searching for H⁺₃ in emission may well provide a better observational handle on this chemically active and important species. The role of proton in hopping in the thermalisation of the H⁺₃ortho/para ratio will be discussed.