  Although defects on graphene can degrade electron transport and its ability
for use as a protection layer, they can also be helpful to tailor the local
properties or activate new sites for particular adsorbates. Here, carbon
vacancy defects are formed in graphene films on Ru(0001) using low energy
Ar$^+$ bombardment and the materials are then reacted at room temperature with
oxygen (O$_2$). Helium low energy ion scattering shows that no oxygen attaches
to the intact graphene layer. When isolated single carbon vacancy defects are
present, oxygen adsorbs molecularly at the defect sites and intercalates
beneath the graphene overlayer after post-annealing at 600 K. When the defects
are large enough to consist of open areas of bare substrate, the oxygen
dissociatively chemisorbs to the Ru. This work shows that the adsorption
depends on the size of the surface vacancies, and that it is important to have
defect-free graphene when using it as a protection layer.
