  Understanding how foreign chemical species bond to atomic vacancies in
graphene layers can advance our ability to tailor the electronic and magnetic
properties of defective graphenic materials. Here we use ultra-high vacuum
scanning tunneling microscopy (UHV-STM) and density functional theory to
identify the precise structure of hydrogenated single atomic vacancies in a
topmost graphene layer of graphite and establish a connection between the
details of hydrogen passivation and the electronic properties of a single
atomic vacancy. Monovacancy-hydrogen complexes are prepared by sputtering of
the graphite surface layer with low energy ions and then exposing it briefly to
an atomic hydrogen environment. High-resolution experimental UHV-STM imaging
allows us to determine unambiguously the positions of single missing atoms in
the defective graphene lattice and, in combination with the ab initio
calculations, provides detailed information about the distribution of
low-energy electronic states on the periphery of the monovacancy-hydrogen
complexes. We found that a single atomic vacancy where each sigma-dangling bond
is passivated with one hydrogen atom shows a well-defined signal from the
non-bonding pi-state which penetrates into the bulk with a (\sqrt 3 \times
\sqrt 3)R30^ \circ periodicity. However, a single atomic vacancy with full
hydrogen termination of sigma-dangling bonds and additional hydrogen
passivation of the extended pi-state at one of the vacancy's monohydrogenated
carbon atoms is characterized by complete quenching of low-energy localized
states. In addition, we discuss the migration of hydrogen atoms at the
periphery of the monovacancy-hydrogen complexes which dramatically change the
vacancy's low-energy electronic properties, as observed in our low-bias
high-resolution STM imaging.
