Near-infrared spectroscopy of above the barrier to linearity

Abstract

Since the Royal Society Discussion Meeting on H3+ in 2000, the laboratory spectroscopy of H3+ has entered a new regime. For the first time, transitions of H3+ above the barrier to linearity have been observed. A highly sensitive near-infrared spectrometer based on a titanium:sapphire laser and incorporating a dual-beam, double-modulation technique with bidirectional optical multi-passing has been developed in order to detect these transitions, which are more than 4600 times weaker than the fundamental band. We discuss our recent work on the 2v1 + 2v2(2) <-- 0, 3v1 + v2(1) <-- 0, v1 + 4v2(2) <-- 0, v1 + 4V2(4) <-- 0 and 2v1 + 3v2(1) <-- 0 combination bands and the 5v2(1) <-- 0, 5v2(3) <-- 0, 52(5) <-- 0 and 6v2(2) <-- 0 overtone bands. Experimentally determined energy levels provide a critical test of ab initio calculations in this challenging energy regime (greater than 10,000 cm(-1)). By comparing the experimental energy levels and theoretical energy levels from ab initio calculations in which the adiabatic and relativistic corrections are incorporated, the extent of higher-order effects such as non-adiabatic and radiative corrections is revealed.

On the Symmetry and Degeneracy of H3+

“…As the simplest polyatomic molecule, H 3 + serves as a useful benchmark for quantum chemistry, and its extensive laboratory spectroscopy 2 (especially above the barrier to linearity 3−7 ) has helped spur advances in ab initio methods 8−11 that now calculate H 3 + energy levels to within ∼0.1 cm −1 of experimental accuracy, even at high energies. 12, 13 The dissociative recombination (DR) of H 3 + with an electron is the simplest possible polyatomic DR process, 14,15 and highresolution spectroscopy has helped to address the ongoing fundamental questions about its nuclear-spin-dependent recombination rates. 16−23 Spectroscopy of H 3 + has also profoundly impacted the astrophysics community.…”

“…H 3 + had long been known to be the initiator of ion−molecule chemistry in the interstellar medium, 24,25 but the infrared spectrum was ultimately what enabled its detection in the interstellar medium 26−28 as well as in planetary atmospheres. 29−31 In interstellar clouds, H 3 + is measured in absorption, facilitating direct estimates of its abundance, and owing to its simple chemistry, the abundance can be related to the cosmic ray ionization rate in a direct fashion. Using this method, subsequent observations of H 3 + have established that the cosmic ray ionization rate in diffuse molecular clouds is an order of magnitude greater than previously thought.…”

“…29−31 In interstellar clouds, H 3 + is measured in absorption, facilitating direct estimates of its abundance, and owing to its simple chemistry, the abundance can be related to the cosmic ray ionization rate in a direct fashion. Using this method, subsequent observations of H 3 + have established that the cosmic ray ionization rate in diffuse molecular clouds is an order of magnitude greater than previously thought. 32,33 H 3 + is also seen in sight lines toward the Galactic center, where it is used as a probe of temperature and density.…”

“…42−44 However, beyond the fundamentally intriguing chemical physics, this reaction is likely the most commonly occurring bimolecular reaction in the Universe. It plays a key role in establishing the "nuclear spin temperature" of H 3 + in diffuse molecular clouds, 45 and the partially deuterium-substituted versions of the H 3 + + H 2 reaction have the greatest influence on deuterium fractionation in dense clouds and prestellar cores. 46−48 The applications mentioned above involve spectroscopically measuring the relative abundances of the two nuclear spin modifications of H 3 + .…”

Communication: Visible line intensities of the triatomic hydrogen ion from experiment and theory

“…The first measurements just above this barrier to linearity were performed by Gottfried and coworkers using advanced absorption spectroscopy. 17,18 This method reaches its limit at 13 700 cm −1 , where the absorption Einstein coefficient B of the transitions reaches a few 10 18 cm 3 J −1 s −2 . No Einstein coefficients could be derived due to unknown initial population of the H + 3 levels.…”