Rotational excitation of NS+ by H2 revisited: A new global potential energy surface and rate coefficients

Abstract

Due to the lack of specific collisional data, the abundance of NS+ in cold dense interstellar clouds was determinedusing collisional rate coefficients of CS as substitute. To better understand the chemistry of sulfur in the interstellarmedium, further abundance modeling using the actual NS+ collisional rate coefficients are needed. For this purpose,we have computed the first full 4D potential energy surface of the NS+−H2 van der Waals complex using the explicitly correlated coupled cluster approach with single, double, and non-iterative triple excitation in conjunction withthe augmented-correlation consistent-polarized valence triple zeta basis set. The potential energy surface exhibitsa global minimum of 848.24 cm−1 for a planar configuration of the complex. The long-range interaction energy, described using multipolar moments, is sensitive to the orientation of H2 up to radial distances of ∼ 50 a0. From thisnew interaction potential, we derived excitation cross sections, induced by collision with ortho- and para-H2, for the15 low-lying rotational levels of NS+ using the quantum mechanical close-coupling approach. By thermally averagingthese data, we determined downward rate coefficients for temperatures up to 50 K. By comparing them with the previous NS+−H2 data, we demonstrated that reduced dimensional approaches are not suited for this system. In addition,we found that the CS collisional data underestimate ours results by up to an order of magnitude. The differences clearlyindicate that the abundance of NS+, in cold dense clouds retrieved from observational spectra, must be reassessed usingthese new collisional rate coefficients.

An accurate set of H3O+ − H2 collisional rate coefficients for non-LTE modelling of warm interstellar clouds

“…In accordance with the results of previous authors, the cross sections we computed with the ortho-H 2 projectile are somewhat larger compared to the para-H 2 data, but the differences usually do not exceed 10%. It is worth noticing that for some transitions we observe more pronounced differences (up to about 20 − 30%) at higher energies (⩾ 400 cm −1 ), which is different from what was generally found for other ion + molecule collisions, where the main differences were present in the low-energy regime (see, for example, the works by Bop et al (2022) and Denis-Alpizar et al (2022)). This feature is most probably associated to the larger anisotropy of the PES with respect to H 2 rotation in the short range.…”

“…Our further important finding following the analysis of cross sections is that the data obtained for the 𝑝−H 2 and 𝑜−H 2 projectiles are very similar. This is an expected behaviour, and the same tendency was observed earlier for several other 'ion + molecular hydrogen' collision schemes, for example recently for NS + + H 2 (Bop et al 2022) (Kłos & Lique 2011). According to Lara-Moreno et al (2019) these similarities between the 𝑝−H 2 and 𝑜−H 2 cross sections could be attributed to the features of the short-range interaction of the colliders, which gives relevant contributions in the coupling matrix elements equally for 𝑝−H 2 and 𝑜−H 2 .…”

Collisional excitation of C2H− by H2: New interaction potential and scattering calculations

“…This highlights that, for this system, the use of a 2D-PES obtained by an average over three H 2 orientations does not allow to obtain results with an accuracy better than 30%. Bop et al 29 obtain a similar conclusion in the study of rotational excitation of NS + by para-H 2 . This shows the need for using a PES describing all orientations of H 2 in cases where the PES exhibits large anisotropies at R-distances dominant for the nuclear dynamics.…”

“…28 However, because of the strong anisotropy of the interaction PES with respect to the orientation of H 2 usually found for ionic species, the use of a reduced dimension PES could lead to inaccuracies in the determination of para-H 2 rate coefficients, as found recently for NS + -H 2 collisions. 29 Moreover, calculations with a 2D-PES do not allow to obtain the data for collisions with ortho-H 2 , much needed for the non-LTE modeling of media where both para-and ortho-H 2 are considered important colliders.…”

BASECOL2023 scientific content

“…The de-excitation among rotational levels of NS + cation by H 2 has been included with several datasets: a first dataset (Bop 2019; 24 levels; T = 5-100 K) of NS + in collision with p-H 2 ( j = 0) that was calculated with a PES spherically averaged over the H 2 directions, and two datasets (Bop et al 2022a; 15 levels; T = 5-50 K) in collision with p-H 2 ( j = 0) and with o-H 2 ( j = 1); both datasets were calculated with a 4D PES (Bop et al 2022a). For these datasets, the authors (Bop et al 2022a) performed some interesting precision tests related to the dimension of the H 2 rotational basis in the dynamical calculations. They found that the neglect of higher H 2 rotational levels induced differences up to 30% in the rate coefficients.…”