The formation of H2 on interstellar dust

We consider the process of H2 formation on realistic dust grain materials. We argue that formation on amorphous H2O ice and polymeric carbonaceous materials leaves H2 in a highly excited vibrational state, while formation on aromatic carbonaceous material gives H2 in states of low excitation. We calculate the H2 vibrational emission intensity arising from H2 formation in dark clouds fromwhich UV radiation is excluded. We show that this emission may be detectable with present technology, and should certainly be detectable at very high spectral resolution. Formation of H2 on amorphous ice leads to local heating which may desorb local CO. We propose that the observed extinction threshold for solid CO is controlled by this process, and show that it leads to CO deposition in distinct polar and non-polar, environments, as observed. Observational consequences of this prediction are described.