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Grain-surface reactions play an essential role in interstellar chemistry,since dust grain catalyses reactions at its surface allowing for the formationof molecules. We used a chemical model in which both gas-phase andgrain-surface reactions occur and studied particularly the diffusion mechanismson the surface of the grains. Surface reactions can occur via thermal hoppingwhen species cross over a potential barrier or via quantum tunneling whenspecies cross through this barrier. We show that the thermal diffusion(hopping) can be much more efficient after a cosmic ray particle collides witha dust grain, heating it to a peak temperature of 70K. We present here theresults of numerical simulations after including the quantum tunnelingmechanism for species H, H2 and O and considering the effect of cosmic rayparticle collision on the surface reactions. As a consequence, the gas-phaseand grain-surface abundances are affected and we show that more complexmolecules can be formed in molecular clouds.

Grain Surface Reactions in Molecular Clouds: The Effect of Cosmic Rays and Quantum Tunneling