The mobility of species adsorbed on the surfaces of interstellar grains: consequences for the formation of interstellar molecules

A fundamental parameter in determining the efficiency of surface reactions on interstellar grains is the mobility with which adsorbed atoms and molecules move on the surface to locate either chemically active sites or another adsorbed species. This surface mobility is calculated here in a fully quantum mechanical treatment for a general atom–surface interaction. The results are expressed as a contour plot in terms of a scaled grain temperature and a scaled height of the energy barrier to motion across the surface. Given these values, the mobility in simple representative cases can be read off the diagrams provided. The results show that hydrogen atoms, either physisorbed or chemisorbed, are mobile, and normally explore the grain surface within a short timescale. Physisorbed heavy atoms are likely also to be mobile, but much less so than for hydrogen. We conclude, therefore, that only hydrides will form by the simple mechanism of interaction of physisorbed species. Chemisorbed heavy atoms are essentially localized. Surface reactions which produce molecules with two or more heavy atoms require, therefore, mechanisms other than the simple two-dimensional kinetic model.