HeH⁺ Collisions with H₂: Rotationally Inelastic Cross Sections and Rate Coefficients from Quantum Dynamics at Interstellar Temperatures

Abstract

We report for the first time an accurate ab initio potential energy surface for the HeH + –H 2 system in four dimensions (4D) treating both diatomic species as rigid rotors. The computed ab initio potential energy point values are fitted using an artificial neural network method and used in quantum close coupling calculations for different initial states of both rotors, in their ground electronic states, over a range of collision energies. The state-to-state cross section results are used to compute the rate coefficients over a range of temperatures relevant to interstellar conditions. By comparing the four dimensional quantum results with those obtained by a reduced-dimensions approach that treats the H 2 molecule as an averaged, nonrotating target, it is shown that the reduced dimensionality results are in good accord with the four dimensional results as long as the HeH + molecule is not initially rotationally excited. By further comparing the present rate coefficients with those for HeH + –H and for HeH + –He, we demonstrate that H 2 molecules are the most effective collision partners in inducing rotational excitation in HeH + cation at interstellar temperatures. The rotationally inelastic rates involving o -H 2 and p -H 2 excitations are also obtained and they turn out to be, as in previous systems, orders of magnitude smaller than those involving the cation. The results for the H 2 molecular partner clearly indicate its large energy-transfer efficiency to the HeH + system, thereby confirming its expected importance within the kinetics networks involving HeH + in interstellar environments.

Rotational Excitation of NCCN by p-H2(jc = 0) at Low Temperatures

“…Table 3 shows the ratio of excitation rates of NCCN−H 2 with both scaled and non-scaled NCCN−He rates for various rotational excitations at 50 K. The trend across various temperatures largely remains the same, and the results are synonymous with previous work published by Giri et al on HeH + −H 2 collision. 27 The scaling results show that S f scaled rates for He perform marginally better at low temperatures (∼10 K) for low-order excitations, i.e., Δj = 2. The scaling of He excitation rates largely remains independent of the temperature thereafter.…”

“…The study of cold and ultracold collisions of NCCN with the He atom has already been carried out in the past by our group. , While rate coefficients for these collisions with He can be roughly compared to the collision with p -H 2 by employing a scaling factor ( S f ), such approximations have previously been deemed unreliable. , In the present paper, rate coefficients for various rotational excitations of NCCN in the presence of p -H 2 ( j c = 0) are calculated using Maxwell–Boltzmann velocity distribution. These rate coefficients are then compared to the previous results, i.e., NCCN–He, for temperatures ranging from 1 to 100 K. The validity of the scaling factor for the He collision rate is also tested for NCCN.…”

Quantum Inelastic Scattering of ArHAr+, HeHHe+, and NeHNe+ with He on New Potential Energy Surfaces

“…Several molecules containing the He atom have also been suggested to be possibly detectable in the ISM (e.g., HeH 3 + and CHe 2+ ; HeHHe + , HeHNe + , and HeHAr + ; and H 2 He + ). The collisional rates for the collision of HeH + with He, H, and H 2 have already been reported. − Furthermore, very recently, the collision of HeHHe + with He was studied for the first time.…”

Rotational State-to-State Rate Coefficients of HeHNe+ by Collision with He at Low Temperatures

“…The rate coefficients for noble-atom-bearing molecules have be computed for a limited number of systems, e.g., ArH + + He, 21,22 ArH + + H, 23 HeH + + He, 24 HeH + + H, 25 HeH + + H 2 , 26 NeH + + He, 27 and HeHHe + + He. 28 We recently reported the rates for NeHNe + + He and ArHAr + He + for the first time and extended the data for HeHHe + + He.…”