Measurements of molecular hydrogen formation on carbonaceous grains

Measurements of molecular hydrogen formation on an amorphous carbon sample have been carried out under astrophysically relevant conditions, i.e., using low fluxes of cold (∼200 K) hydrogen-isotope beams, sample temperature between 5 K and 20 K. The experiments were conducted using the same methods as the ones employed for similar measurements on an olivine sample; therefore, results from the two experiments can be compared to each other. As in the case for the study of hydrogen recombination on a silicate surface, we find that mobility of hydrogen is thermally activated. However, differently from the olivine case, the activation energy for the overall process of diffusion and desorption is significantly higher, and the recombination efficiency, at similar total fluence of impinging atoms and sample temperature during irradiation, is also higher. Implications of these measurements for astrophysical environments are discussed.