Near-Infrared to Visible Spectral Lines of Cold H_3^+

Details Near-Infrared to Visible Spectral Lines of Cold H_3^+ Near-Infrared to Visible Spectral Lines of Cold H_3^+

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.299p&link_type=abstract

The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole

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Scientific paper

The triatomic hydrogen ion H_3^+ is the cornerstone of interstellar chemistry. It's properties and reactional behavior at cold temperatures can give important insight in amongst other isotopic fractionation and pathways to the production of larger molecules. As simplest polyatomic molecule, it is also a pivot theoretical benchmark molecule. Especially the highly excited region of H_3^+ is an important benchmark towards a treatment for strongly distorted molecular symmetries and non-adiabatic interactions. We have investigated the H_3^+ ion in a cryogenic 22-pole ion trap using chemical probing spectroscopy with argon. The H_3^+ ions are buffer-gas cooled by Helium to interstellar temperatures populating only the lowest three rotational levels {J,G}=(1,1) and (2,2) of para symmetry and (1,0) of ortho symmetry. The level populations may be changed by the set trap temperature or by manipulating the ortho-to-para ratio with the use of para-H_2 parent gas. As the reaction with argon is endothermic, only the laser excited H_3^+ will react and form ArH^+ which is used as highly sensitive spectroscopic signal. We have measured near-infrared to visible spectral lines of H_3^+ giving, as a first, transition frequencies and probabilities for transitions that are up to 10^6 times weaker than the fundamental transition. Energy levels up to 17000 cm-1 were determined, leading to a correction improving predictions to spectroscopic accuracy in this region. Moreover, our measurements agree well with the expected intensities of the bulk states. The strong intensities predicted for excitation to highly excited bending states, called Horseshoe states, however, are surprisingly not found. Further investigation, both in theory and in experiment, is required. Our spectroscopy technique has not yet reached its sensitivity limit, paving the way to the investigation of forbidden infrared lines and of excitation closer to and beyond the dissociation limit.

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