  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, which mechanism 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+O$_3$$\rightarrow$O$_2$+OH on the surface of a film
of non-porous amorphous solid water (np-ASW) in the temperature range from 10 K
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 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.
