Mechanism of Atomic Hydrogen Addition Reactions on np-ASW

Hydrogen, being the most abundant element, is the driver of many if not most reactions occurring on interstellar dust grains. In hydrogen atom addition reactions, the rate is usually determined by the surface kinetics of the hydrogen atom instead of the other reaction partner. Three mechanisms exist to explain hydrogen addition reactions on surfaces: Langmuir-Hinshelwood, Eley-Rideal, and hot-atom. In gas-grain models, the mechanism that is assumed greatly affects the simulation results. In this work, we quantify the temperature dependence of the rates of atomic hydrogen addition reactions by studying the reaction of H+O₃ → O₂+OH on the surface of a film of non-porous amorphous solid water (np-ASW) in the temperature range from 10 to 50 K. The reaction rate is found to be temperature independent. This disagrees with the results of simulations with a network of rate equations that assume Langmuir-Hinshelwood mechanism through either thermal diffusion or tunneling diffusion; the reaction rates assuming such a mechanism possesses a strong temperature dependence, either explicitly or implicitly, that is not seen experimentally. We suggest that the Eley-Rideal and/or hot-atom mechanism play a key role in hydrogen atom addition reactions, and should be included in gas-grain models. We also suggest that our newly developed time-resolved reactive scattering can be utilized to measure the chemical desorption efficiency in grain surface reactions.