Formation of interstellar SH⁺from vibrationally excited H₂: Quantum study of S⁺+ H₂⇄ SH⁺+ H reaction and inelastic collision

Abstract

The rate constants for the formation, destruction, and collisional excitation of SH + are calculated from quantum mechanical approaches using two new SH + 2 potential energy surfaces (PESs) of 4 A and 2 A electronic symmetry. The PESs were developed to describe all adiabatic states correlating to the SH + ( 3 Σ − ) + H( 2 S ) channel. The formation of SH + through the S + + H 2 reaction is endothermic by ≈ 9860 K, and requires at least two vibrational quanta on the H 2 molecule to yield significant reactivity. Quasi-classical calculations of the total formation rate constant for H 2 (v = 2) are in very good agreement with the quantum results above 100K. Further quasi-classical calculations are then performed for v = 3, 4, and 5 to cover all vibrationally excited H 2 levels significantly populated in dense photodissociation regions (PDR). The new calculated formation and destruction rate constants are two to six times larger than the previous ones and have been introduced in the Meudon PDR code to simulate the physical and illuminating conditions in the Orion bar prototypical PDR. New astrochemical models based on the new molecular data produce four times larger SH + column densities, in agreement with those inferred from recent ALMA observations of the Orion bar.

Hyperfine excitation of SH+ by H

“…The reaction dynamics on the 4 A state was studied with a time-independent treatment based on hyperspherical coordinates. On this PES, the SH + + H → H 2 + S + collision is a barrierless and exothermic reaction for which it has been shown (Zanchet et al 2019) that the reactivity is strong (k > 10 −10 cm 3 s −1 ), even at low temperatures. The competition between all the three processes (inelastic, exchange, and reactivity) is therefore rigorously taken into account.…”

“…The collisions between SH + (X 3 Σ − ) and H( 2 S ) can take place on two different potential energy surfaces (PESs), the ground quartet ( 4 A ) and doublet ( 2 A ) electronic states of the H 2 S + system. In this work, we use the H 2 S + quartet and doublet PESs that were previously generated by Zanchet et al (2019).…”

“…We computed the cross sections for the first 13 rotational levels of the SH + molecule (0 < N < 12) for a collisional energy ranging from 0 to 5000 cm −1 and for all values of the total angular momentum J leading to a nonzero contribution in the cross sections. More details about the scattering calculations can be found in Zanchet et al (2019).…”

“…SH + rotational lines have also been detected in emission toward the Orion Bar photodissociation region (PDR; Nagy et al 2013), a strongly UV-irradiated surface of the Orion molecular cloud (e.g., Goicoechea et al 2016). In warm and dense PDRs (n H 10 5 cm −3 ) such as the Orion Bar, SH + forms by exothermic reactions of S + with vibrationally excited H 2 (with v ≥ 2, see details in Agúndez et al 2010;Zanchet et al 2013Zanchet et al , 2019. High angular resolution images taken with ALMA show that SH + arises from a narrow layer at the edge of the PDR, the photodissociation front that separates atomic from molecular gas (Goicoechea et al 2017).…”

“…Collisional data with atomic hydrogen are much more challenging to compute because of the possible reactive nature of the SH + -H collisional system. However, we have recently overcome this difficult problem and presented quantum mechanical calculations of cross sections and rate coefficients for the rotational excitation of SH + by H, including the reactive channels (Zanchet et al 2019), using new accurate potential energy surfaces. Unfortunately, it was not possible to include the fine and hyperfine structure of the SH + ( 3 Σ − ) molecule in the quantum dynamical calculations, but they are resolved in astronomical observations.…”