Rotational Transitions of HOC⁺ Induced by Collision with He at Low Temperatures

Abstract

The HOC + molecule has for long been detected in several regions of the interstellar medium (ISM). The collisional ro-vibrational rate coefficients of this molecule with the most common colliders in the ISM are then required for applying nonlocal thermal equilibrium models. However, this molecule has a low bending frequency (249 cm −1 ), and the use of the rigid rotor approximation is therefore limited to low collision energies. Also, the complete determination of the ro-vibrational rate coefficients of HOC + in collision with He requires including the bending motion in the analytical model of the potential energy surface (PES) of the system. The first goal of this work is then to develop the first rigid bender fourdimensional PES for the interaction between HOC + and He. To this aim, a large grid of ab initio energies are computed at the CCSD(T)-F12b/aug-cc-pVQZ level of theory and an analytical representation of the PES is obtained using a combination of least square and reproducing kernel Hilbert space procedures. The global minimum of this PES is found to be reached for a linear configuration of the complex. In the second part of this study, rigid rotor close-coupling calculations are performed at low collision energy, and the calculated rate coefficients are compared to those previously determined for the collisions of He with its HCO + isomer.

Rotational transitions of COH+ and He: New interaction potential, bound states, scattering and pressure broadening cross-sections

“…The comparison of present results with that of HCO + -He 12,17 shows that both isomers have comparable rate (∼ 10 −10 cm 3 molecule −1 s −1 ) and similar behaviour. We have also compared our rate coefficients data with Santander et al 34 in the Figure 5 (right panel), and our data is found to be in the same order with very negligible differences as evident from the Table 5. The small differences are suspected to be due to different welldepth values and different procedures of analytical fitting.…”

“…Cross-sections for 1→0 transitions predominate over 2→0 and 3→0. When compared to recent data of cross-sections of COH + -He 34 , the results are found to be approximately same. The small differences observed may be because of difference in reported well-depth values or B e value that they took into consideration.…”

“…Comparison of rate coefficients (in 10 −10 cm 3 molecule −1 s −1 ) data with the reported work of Santander et al34 …”

“…In 1996, Herbst et al studied the reaction rate between H 2 and COH + in the temperature range 10-300 K 33 . Recently, Denis-Alpizar et al determined the collisional rate coefficients of COH + for the first time in collision with He by starting from the potential energy surface (PES) 34 . They further calculated the cross-sections for only two transitions based on their PES.…”

Non-LTE modelling of cyanoacetylene: evidence for isomer-specific excitation

“…10 2 10 4 10 6 10 8 20 40 10 2 10 4 10 6 10 8 10 2 10 4 10 6 10 8 0.0 0. (Santander et al 2019) have been reported over the last decades. In all these studies, differences ranging from few tenths of a percent up to one order of magnitude were found.…”