Laboratory Synthesis of Molecular Hydrogen on Surfaces of Astrophysical Interest

We report on the first results of experiments to measure the recombination rate of hydrogen on surfaces of astrophysical interest. Our measurements give lower values for the recombination efficiency (sticking probability S x probability of recombination upon H-H encounter $gamma$) than model-based estimates. We propose that our results can be reconciled with average estimates of the recombination rate (1/2 n(H) n(g) v(H)A S $gamma$) from astronomical observations, if the actual surface of an average grain is rougher, and its area bigger, than the one considered in models.