▎ 摘　　要

We study the adsorption of atomic hydrogen on graphene by combining scanning tunneling microscopy experiments and first principle calculations. Our results reveal the existence of a physisorption channel over the graphene layer, dominated by van der Waals forces and thus homogeneous over the whole atomic lattice, where atomic hydrogen can move freely. Such physisorption channel is essential to understand the final configuration of hydrogen atoms chemisorbed on graphene. We find that similar to 95% of chemisorbed H atoms form non-magnetic dimers even for very dilute concentrations (<0.1%) deposited at low temperatures (140 K). Our data shows that this scenario holds from mono to multilayers graphene on SiC(0 0 0 - 1), SiC(0 0 0 1) and graphite.