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

¹ Instituto de Física Fundamental (IFF-CSIC), C.S.I.C., Serrano 123, 28006 Madrid, Spain
e-mail: octavio.roncero@csic.es
² LOMC – UMR 6294, CNRS-Université du Havre, 25 rue Philippe Lebon, BP 1123, 76 063 Le Havre Cedex, France
³ Firat University, Department of Physics, 23169 Elazig, Turkey

Received: 14 March 2019
Accepted: 6 May 2019

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 ⁴A″ and ²A″ electronic symmetry. The PESs were developed to describe all adiabatic states correlating to the SH⁺(³Σ⁻) + H(²S) channel. The formation of SH⁺ through the S⁺ + H₂ reaction is endothermic by ≈9860 K, and requires at least two vibrational quanta on the H₂ molecule to yield significant reactivity. Quasi-classical calculations of the total formation rate constant for H₂(v = 2) are in very good agreement with the quantum results above 100 K. Further quasi-classical calculations are then performed for v = 3, 4, and 5 to cover all vibrationally excited H₂ 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.