Ortho–para-H₂conversion processes in astrophysical media

Abstract

We report in this review recent fully quantum time-independent calculations of cross sections and rate constants for the gas phase ortho-to-para conversion of H 2 by H and H + . Such processes are of crucial interest and importance in various astrophysical environments. The investigated temperature ranges were 10-

C-type shock modelling – the effect of new H2–H collisional rate coefficients

“…The treatment of ortho-/para-H 2 interconversion was usually adopted according to Le Bourlot et al (1999), that in turn is based on the results of QCT calculations by Mandy & Martin (1993); Martin & Mandy (1995) and laboratory experiments (Schulz & Le Roy 1965;Schofield 1967). Until recently, the contribution of the reactive scattering channels to the H 2 excitation by H atoms remained a significant source of uncertainty, see discussion by Wrathmall et al (2007); Lique et al (2014). Lique (2015) reported time-independent quantum mechanical calculations of rate coefficients for the collisional (de-)excitation of H 2 by H atoms.…”

The H+ + H2reaction

“…Despite the vast list of existing studies on the subject, this work does not contain any explicit reference to preceeding work to calculate potential energy surfaces (PESs) of the H + 3 system. The interested reader is referred however to the recent review by Lique et al [1]. Suffice it to say for the scope of this review that most of the theoretical investigations on the reaction carried out by the authors employed PESs from References [2][3][4].…”

Rovibrational transitions of H 2 by collision with H + at high temperature

“…The spin-scrambling mechanisms to convert o-H 2 into p-H 2 require the reactive collision of the molecular hydrogen with H, H 2 , H + or H + 3 . Due to the existence of noticeable barriers in those reactions with the neutral species, at low temperature the ortho-para conversion is mainly mediated by the H + +H 2 ( j odd) → H 2 ( j even) + H + and H + 3 + H 2 ( j odd) → H 2 ( j even) + H + 3 processes (Lique et al 2014). Although in principle radiative processes and rotational excitation collisions should preserve the para/ortho character of molec-ular hydrogen according to total parity conservation considerations, the complex-forming dynamics of the H + +H 2 reaction at low energies have been invoked to explain the redistribution of the H 2 ( j) rotational states (Roueff et al 1988).…”

“…At high temperature, neutral hydrogen exchange is also a possible way to convert o-H 2 into p-H 2 . In this sense, and due to its relevance in astrophysical media, the temperature dependence of rate constants for ro-vibrational excitation of molecular hydrogen from different initial H 2 (v, j) states have been analysed (Lique et al 2012;Lique 2015;Lique et al 2014). New results ranging from 100 to 5000 K suggest a possible review of the commonly assumed cooling mechanism for several astrophysical media (Lique 2015).…”