Monte Carlo simulations of H2 formation on grains of varying surface roughness

Continuous-time random-walk Monte Carlo simulations of H2 formation on a variety of grain surfaces of varying roughness based on olivine and amorphous carbon have been performed. With these inhomogeneous surfaces, we find that the temperature range over which efficient H2 formation occurs in the interstellar medium is larger than it is for flat surfaces characterized by single values of the energy parameters for hydrogen-atom adsorbates. Our results show, in particular, that the formation of H2 on non-flat interstellar grains can occur efficiently at typical surface temperatures in diffuse interstellar clouds, although the results are dependent on the strength of lateral bonds between H atoms and the surface.