Reactions of the hydrogen radical family (H, OH, HO2) are significant in Earth’s atmosphere, combustion, as well as in extraterrestrial environments. Here we present investigations of the kinetics of two reactions, the OH + CO and H + HO2, which play important roles in the interstellar medium and on Mars. We have measured the rate coefficient OH + CO reaction at low temperatures by the CRESU (Cinétique de Réactions en Ecoulement Supersonique Uniforme) method. The new measurements show that the reaction rate, which decreases with temperature down to 50 K, increases as the temperature is further lowered to 30 K. We compare these results to theoretical predictions from Semi-Classical Transition State Theory (SCTST) and Ring Polymer Molecular Dynamics (RPMD). We discuss the implications of the turnaround in the rate coefficient for the interstellar abundance of CO2. Water is thought to have been abundant on Ancient Mars, but a leading hypothesis is that the water escaped from the atmosphere to space over 3.5 Gy by transport of H2 to the upper atmosphere, where H2 dissociation led to H-atom escape. The only gas phase source of molecular hydrogen in the Mars atmosphere is a minor channel in the H + HO2 reaction. We have performed theoretical calculations of the rates and branching ratios for all channels. We find that the temperature dependence, product branching ratios, and Deuterium Kinetic Isotope Effect (DKIE) disagree with earlier experiments (no DKIE has yet been measured). A new photochemical model is developed to explore the implications for predictions of water escape from Mars.

Kinetics of hydrogen radical reactions in extraterrestrial environments

ACS Spring 2022

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