Evidence of hydrogenated hexamers on graphite

Hydrogenated clusters on graphite are known to form stable long-range oriented star structures when observed by scanning tunneling microscopy as recently shown by Hornekوr et al. [L. Hornekوr, W. Xu, R. Otero, E. Lوgsgaard, F. Besenbacher, Chem. Phys. Lett. 446 (2007) 237]. The atomic arrangement of these clusters has remained unknown up to now. We herein assigned the star structures to hydrogenated hexamers on the surface of graphite, by means of density functional theory calculations. We found these structures to be stable; the calculated STM images we inferred from the calculations are in excellent agreement with experimental ones.