Using state-of-the-art ab initio interaction-induced dipole and potential-energy surfaces for hydrogen–helium (H2–He) pairs, we compute the rototranslational collision-induced absorption coefficient at 40-400 K for frequencies covering 0-4000 cm−1. The quantum mechanical scattering calculations account for the full anisotropic interaction potential, replacing the isotropic approximation. The absorption data are expected to be accurate with an uncertainty of 2% or better up to 2500 cm−1. The uncertainty is slightly higher at the highest frequencies where the rototranslational absorption is largely obscured by the rovibrational band. Our improved agreement with measurements at 200-800 cm−1 result from the improvement of the potential energy surface. The previously available rototranslational data set for H2–H2 pairs (Fletcher et al., Astrophys. J. Supp. 235, 24 (2018)) is also extended up to 4000 cm−1. In the rovibrational band previous isotropic potential calculations for H2–He (Gustafsson et al., J. Chem. Phys. 113, 3641 (2000)) and H2–H2 (Borysow, Icarus 92, 273 (1992)) have been extended to complement the rototranslational data set. The absorption coefficients are tabulated for ortho-to-para ratios from normal-H2 to pure para-H2, as well as equilibrium-H2, over 40-400 K . The effect of these updates are simulated for the cold atmosphere of Uranus and warmer atmosphere of Jupiter. They are equivalent to a brightness temperature difference of a fraction of a degree in the rototranslational region but up to 4 degrees in the rovibrational region. Our state-of-the-art modifications correct an otherwise +2% error in determining the He/H2 ratio in Uranus from its spectrum alone.

Line shape parameters of the first pure rotational R(j = 0 − 4) lines of carbon monoxide in hydrogen baths, between 1 and 500 K, have been calculated using the close coupling method. The theoretical thermally averaged collisional widths and shifts between 30 and 500 K agree well with the values reported in the literature. However, below this temperature range, we confirm the long-standing substantial disagreement between experimental and theoretical values for the R(0) and R(1) lines. In addition to the usual collisional widths and shifts, we provide the complex optical frequency of the velocity-changing collisions. We also study the speed dependence of the line shape parameters and investigate their double power law temperature representation. We conclude that beyond-Voigt effects, including the collision duration, cannot reconcile theory and experiments at low temperatures.

Close coupling calculations of line shape parameters have been performed for the first pure rotational R0(j = 0–4) lines of CO in helium baths at various temperatures. Besides the usual Lorentzian widths and shifts, we provide the complex Dicke parameters as well as the double power law temperature representation of all four parameters. In addition, we study the speed dependence of these parameters. The R0(0) and R0(1) theoretical thermally averaged collisional widths and shifts between 500 and about 15 K are in excellent agreement with the values reported in the literature. Below this temperature range, we confirm the persistent substantial disagreement that exists since 1985 between experimental and theoretical values. We thus focus on this regime, which is important for astrophysical applications, and we discuss various beyond-Voigt effects at low temperatures to try to understand this mismatch. We show that such mechanisms do not allow experimental widths and shifts to be reconciled with those from theory.  

Accurate collision-induced absorption profiles for H2–He pairs, in the rototranslational band of H2, are computed accounting for the full anisotropic interaction potential. The calculations are time consuming and complicated compared to those pursued in the isotropic potential approximation. A machine learning approach is implemented in order to produce highly accurate data on a dense frequency grid, by combining data computed in the full calculation with those from the isotropic approximation. Thus an extensive, highly accurate, data base can be obtained for a set of frequencies, temperatures, and ortho-H2/para-H2 fractions, appropriate for use in modeling of planetary atmospheres, in particular of the gas giants.

We have developed the polyatomic extension of the established [M. Gustafsson, J. Chem. Phys. 138, 074308 (2013)] classical theory of radiative association in the absence of electronic transitions. The cross section and the emission spectrum of the process is calculated by a quasiclassical trajectory method combined with the classical Larmor formula which can provide the radiated power in collisions. We have also proposed a Monte Carlo scheme for efficient computation of ro-vibrationally quantum state resolved cross sections for radiative association. Besides the method development, the global potential energy and dipole surfaces for H + CN collisions have been calculated and fitted to test our polyatomic semiclassical method.

Empirical Barker-Fisher-Watts and modified Tang-Toennies potential energy curves are obtained through fit to experimental vibrational transition energies for argon–argon, xenon–xenon, and argon–xenon pairs. The potentials are tested against experimental thermophysical and transport properties, and agreement is observed. Also, an interaction-induced electric dipole moment curve for the argon–xenon pair is determined through a fit to experimental spectral moments for collision-induced absorption. The argon–xenon potentials and dipole are tested in a complete quantum dynamical calculation of the collision-induced absorption profiles, which can be compared with a laboratory measurement. This provides further analysis of the accuracy of the empirical argon–xenon data, as calculations of absorption profiles are highly sensitive to the input of molecular data.

Context. Radiative association is a possible way of sodium chloride (NaCl) formation in interstellar and related environments. Theoretical studies are essential since laboratory experiments are unavailable and difficult to perform.

Aims. The total rate coefficient was calculated for the formation of NaCl by radiative association at 30–750 K.

Methods. We included two contributing processes for the total rate-coefficient computation. One of them takes the nonadiabatic coupling between the two lowest ¹Σ+ states, Χ¹Σ+ and Β¹Σ+, into account. The other one was calculated conventionally as a single channel and started in the continuum of the A¹Π state. The individual rate coefficients were calculated from cross sections obtained up to 0.8 eV, which enabled us to calculate the rate coefficients up to 750 K. The cross section was also calculated for a one-state process within the Χ¹ Σ+ state.

Results. The nonadiabatic coupling enhances the formation of NaCl by radiative association by two orders of magnitude at about 30 K and by around one order of magnitude at about 750 K. The single-channel process starting in the continuum of the A¹ Π state starts to contribute above around 200 K. The one-state transition model, within the Χ¹Σ+ state, is not an adequate approximation for collisions in ¹Σ+ symmetry. Instead, these collisions are treated in the diabatic representation in the total rate-coefficient calculation.

Conclusions. The calculated total rate-coefficient function at 30–750 K can improve the astrochemical reaction networks for the CRL 2688, IRC+10216, and Orion SrcI environments, where NaCl was detected before.

Reaction rate constants have been calculated for the formation of CH and CD molecules through radiative association of C and H/D atoms in their ground states. Quantum mechanical and semiclassical/classical methods were used to obtain the reaction cross-sections. Shape resonances and inverse pre-dissociation are accounted for with Breit–Wigner theory. The potential, permanent/transition dipole moment curves and experimental pre-dissociation widths are taken from the literature. The resulting reaction rate constants were fitted to the Kooij formula for use in astrochemical modelling. Our rate constant is 3.5 × 10⁻¹⁷ cm³ s⁻¹ at 100 K and it peaks at 20 K, where it is 8.0 × 10⁻¹⁷ cm³ s⁻¹. These values are larger than what has been obtained in earlier studies but not large enough to account for the interstellar abundance of CH.

Collisions of sodium and chlorine atoms and of their ions are studied within the diabatic two-state picture at energies below and above the ionic threshold with focus on the processes of radiative association, chemiionisation, and mutual neutralisation. The radiative-association cross sections as functions of collision energy are calculated up to 4.6 eV in the case of neutral atoms and up to 3.12 eV in the case of ions. The non-radiative charge-exchange cross sections as functions of collision energy are calculated up to 12 eV for chemiionisation and up to 10.52 eV for mutual neutralisation. The corresponding radiative-association rate coefficients are then determined up to 5300 K for the radiative association of neutral atoms and non-radiative charge-exchange and up to 3615 K for the radiative association of ions. Contribution of many Fano–Feshbach-type resonances is included to the rate coefficient of neutral-atom radiative association. The chemiionisation rate coefficients were calculated from 1000 K to 5300 K. The process of mutual neutralisation exhibits the largest cross sections and also the largest rate coefficients with values around 10⁻⁹ cm³ s⁻¹ at all calculated temperatures, 120–5300 K.