H₂ formation on interstellar grains

The importance of understanding how molecular hydrogen is formed in space is 4-fold. Rapid progress in surface science, and especially in the interaction of atoms or molecules with surfaces, was obtained in the 1980s and 1990s in part due to availability of ultrahigh vacuum techniques and the use of single crystal surfaces for which cleaning and characterization methods could be readily established. Improvements in the application of DFT to atom-surface interaction have led to a more accurate handling of the interaction at large distances where weak dispersion-like forces should dominate. The first involves long-range dispersion forces and the characteristics energies are in the tens to a few hundred meV, while chemisorption is about eV energies and chemical bonding. Obviously, this is just a coarse way to categorize an otherwise wide range of complex interactions.