# The Experimental Rate Constant of the S+(2D) + H2 Reaction

## TL;DR

This study investigates how electronic excitation of sulfur cations enhances the production of SH+ in interstellar environments.

## Contribution

The paper experimentally measures the cross section and derives the rate constant for the S+(2D) + H2 reaction.

## Key findings

- Electronic excitation of S+ significantly enhances reactivity with H2.
- The derived rate constant spans a wide temperature range.
- Results are compared with theoretical predictions between 0.001–3 eV.

## Abstract

Endothermic reactions such as S+(4S) + H2 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.