ROVIBRATIONAL QUENCHING RATE COEFFICIENTS OF HD IN COLLISIONS WITH He

Abstract

Along with H 2 , HD has been found to play an important role in the cooling of the primordial gas for the formation of the first stars and galaxies. It has also been observed in a variety of cool molecular astrophysical environments. The rate of cooling by HD molecules requires knowledge of collisional rate coefficients with the primary impactors, H, He, and H 2. To improve knowledge of the collisional properties of HD, we present rate coefficients for the He-HD collision system over a range of collision energies from 10 −5 to 5 × 10 3 cm −1. Fully quantum mechanical scattering calculations were performed for initial HD rovibrational states of j = 0 and 1 for v = 0-17 which utilized accurate diatom rovibrational wave functions. Rate coefficients of all Δv = 0, −1, and −2 transitions are reported. Significant discrepancies with previous calculations, which adopted a small basis and harmonic HD wave functions for excited vibrational levels, were found for the highest previously considered vibrational state of v = 3. Applications of the He-HD rate coefficients in various astrophysical environments are briefly discussed.

Mapping the Universe in HD

“…[78,79]. For the 1-0 transition we are concerned with here, these rates are in excellent agreement with more recent computations [80][81][82].…”

Line-shape parameters for the first rotational lines of HD in He

“…The H 2 molecule and its isotopologues interacting with helium have been thoroughly investigated in the last decades (for a review see Thibault et al (2016Thibault et al ( , 2017; Wcis lo et al (2018); Martínez et al (2018); Jwiak et al (2018)). For instance, in view of the role played by HD in the astrophysics and astrochemistry of the interstellar media, the rovibrational quenching rate coefficients of HD in collisions with helium (Nolte et al 2011) were calculated using Muchnick and Russek (1994) potential energy surface (PES) and the rotational rate constants were calculated more recently by Zhou and Chen (2017) using the Bakr, Smith, and Patkowski (BSP) PES (Bakr et al 2013). Transport and second virial coefficients were also calculated for the HD-He system (Green et al 1983;McCourt et al 2005;Garberoglio et al 2014).…”

HD (v = 1, j = 2, m) orientation controls HD–He rotationally inelastic scattering near 1 K

“…Presented here are the first measurements that probe the effect of collision geometry on low energy inelastic scattering. Furthermore, this experimental study is the first on low-temperature hydrogen-helium scattering, which is of astrophysical importance [30][31][32] and is also immensely valuable as a test bed for quantum mechanical scattering calculations. [32][33][34][35]…”

“…Calculations on HD-He collisions for a variety of rovibrational de-excitation transitions as well as elastic scattering have predicted several resonances in our collision energy region. 32,[50][51][52] However, none of these studies directly addressed the conditions present in our experiment nor did they examine the stereodynamics of the resonant state. The theoretical study most similar to the experiments presented here revealed a resonance near 0.2 cm −1 for the HD (v = 0, j = 2) → (v ′ = 0, j ′ = 0) rotational relaxation.…”

“…53 A small fraction of the collisions in our experiment do occur with this energy, but it is not possible to make a direct comparison because scattering resonances can depend sensitively on the vibrational state. 32 Figure 6 graphically describes the quantum state controlled scattering experiment presented in this paper. Our ability to prepare HD in a single rovibrational energy eigenstate with m state precision using SARP and to reduce the collision temperature in the range of a few Kelvin allowed us to extract detailed dynamical information on the He-HD atom-diatom collision, a very simple neutral scattering system, using a partial wave analysis of the scattering angular distribution.…”