Formation of molecules on small interstellar grains

A model is proposed for the formation of molecules on small interstellar grains. It is suggested that the energy liberated when a chemical bond is formed between two atoms on a grain is transferred to the lattice vibrations of the grain, heating up the grain. The 'hot' grain, during the time that it is radiatively recooling, may then liberate its adsorbed volatiles. The time scales for molecular desorption from a grain are calculated for OH and CO for four different grain compositions. The results provide some tentative explanations of astronomical observations.