Low-temperature chemistry between water and hydroxyl radicals: H/D isotopic effects

TL;DR

This study investigates the isotopic effects of water and hydroxyl radical reactions at low temperatures, revealing key reaction pathways that influence water isotopic ratios in space through laboratory experiments and simulations.

Contribution

It provides the first systematic experimental comparison of H/D isotopic effects in water-hydroxyl reactions relevant to astrochemistry, combined with kinetic modeling.

Findings

H2O + OD --> OH + HDO occurs, affecting HDO/H2O ratios in space.

D2O + OH reaction is less effective and less relevant astronomically.

Experimental results inform astrochemical models of water formation.

Abstract

Sets of systematic laboratory experiments are presented -- combining Ultra High Vacuum cryogenic and plasma-line deposition techniques -- that allow us to compare H/D isotopic effects in the reaction of H2O (D2O) ice with the hydroxyl radical OD (OH). The latter is known to play a key role as intermediate species in the solid-state formation of water on icy grains in space. The main finding of our work is that the reaction H2O + OD --> OH + HDO occurs and that this may affect the HDO/H2O abundances in space. The opposite reaction D2O + OH --> OD + HDO is much less effective, and also given the lower D2O abundances in space not expected to be of astronomical relevance. The experimental results are extended to the other four possible reactions between hydroxyl and water isotopes and are subsequently used as input for Kinetic Monte Carlo simulations. This way we interpret our findings in an astronomical context, qualitatively testing the influence of the reaction rates.