Metastable structures and recombination pathways for atomic hydrogen on the graphite (0001) surface.

We present scanning tunneling microscopy results which reveal the existence of two distinct hydrogen dimer states on graphite basal planes. Density functional theory calculations allow us to identify the atomic structure of these states and to determine their recombination and desorption pathways. Direct recombination is only possible from one of the two dimer states. This results in increased stability of one dimer species and explains the puzzling double peak structure observed in temperature programmed desorption spectra for hydrogen on graphite.