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The formation of interstellar water has been commonly accepted to occur onthe surfaces of icy dust grains in dark molecular clouds at low temperatures(10-20 K), involving hydrogenation reactions of oxygen allotropes. As a resultof the large abundances of molecular hydrogen and atomic oxygen in theseregions, the reaction H2 + O has been proposed to contribute significantly tothe formation of water as well. However, gas phase experiments andcalculations, as well as solid-phase experimental work contradict thishypothesis. Here, we use precisely executed temperature programmed desorption(TPD) experiments in an ultra-high vacuum setup combined with kinetic MonteCarlo simulations to establish an upper limit of the water production startingfrom H2 and O. These reactants are brought together in a matrix of CO2 in aseries of (control) experiments at different temperatures and with differentisotopological compositions. The amount of water detected with the quadrupolemass spectrometer upon TPD is found to originate mainly from contamination inthe chamber itself. However, if water is produced in small quantities on thesurface through H2 + O, this can only be explained by a combined classical andtunneled reaction mechanism. An absolutely conservative upper limit for thereaction rate is derived with a microscopic kinetic Monte Carlo model thatconverts the upper limit into a maximal possible reaction rate. Incorporatingthis rate into simulations run for astrochemically relevant parameters, showsthat the upper limit to the contribution of the reaction H2 + O in OH, andhence water formation, is 11% in dense interstellar clouds. Our combinedexperimental and theoretical results indicate however, that this contributionis likely to be much lower.

On the relevance of the H2 + O reaction pathway for the surface formation of interstellar water - A combined experimental and modeling study