Grain-surface reactions in molecular clouds: the effect of cosmic rays and quantum tunnelling

Grain-surface reactions play an essential role in interstellar chemistry, since dust grain catalyzes reactions at its surface allowing for the formation of molecules. We used a chemical model in which both gas-phase and grain-surface reactions occur and studied particularly the diffusion mechanisms on the surface of the grains. Surface reactions can occur via thermal hopping when species cross over a potential barrier or via quantum tunnelling when species cross through this barrier. We show that the thermal diffusion (hopping) can be much more efficient after a cosmic ray particle collides with a dust grain, heating it to a peak temperature of 70K. We present here the results of numerical simulations after including the quantum tunnelling mechanism for species H, H-2, and O and considering the effect of cosmic ray particle collision on the surface reactions. As a consequence, the gas-phase and grain-surface abundances are affected and we show that more complex molecules can be formed in molecular clouds.