Shaw et al. 2005, ApJ, 624, 794, describing molecular hydrogen in Cloudy, is available on astro-ph here, or as a high resolution version here.
The first step to form H2 on grains is for the molecule to be captured onto the grain surface. There are several processes by which a gas can be captured by a solid. Physical adsorption (or physisorption) is caused by the van der Waals forces of attraction between molecules of the gas and the atoms or ions that form the surface. Chemisorption occurs due to a chemical bonding of the molecule to the solid - valence electrons are shared to form a chemical bond. A chemisorbed molecule is more tightly bound and difficult to free from the surface than an adsorbed molecule because the forces are stronger. Finally, absorption occurs when the molecule actually penetrates into the solid, forming a solid solution.
last updated June 23, 2005