The Experimental Rate Constant of the S ⁺(² D) + H ₂ Reaction

Endothermic reactions such as S ⁺(⁴ S) + H ₂ are not expected to play a significant role in the chemistry of the interstellar medium (ISM). However, in some specific environments, such as photon-dominated regions (PDR), UV radiation may catalyze the reaction by providing enough internal energy to reactants to overcome endothermicity. For instance, it was recently shown that the vibrational excitation of H₂ greatly enhances the reactivity of C⁺ and S⁺ with H₂, explaining the presence of their respective hydrides CH⁺ and SH⁺ in these regions. However, vibrational excitation of H₂ is not a unique way to enhance the reactivity by UV radiation. Electronic excitation is an alternative way to effectively inject a huge amount of internal energy into the system, thus favoring reactivity. In this work, we will address how electronic excitation of the sulfur cation can strongly enhance the production of SH⁺. This is done by measuring experimentally the cross section of the title reaction for collision energies from 50 meV up to several eV and comparing the results with theoretical predictions in the 0.001-3 eV range. The reaction cross section is then used to derive the rate constant for a wide range of temperatures.