  Graphene is a fascinating 2D material that is being widely investigated for
use in electronic devices due to its unique electronic and materials
properties. Also, because of its high thermal stability and inertness, it is
considered a promising candidate for use as a protection layer for metal
substrates. Here, graphene films grown on Ru(0001) are held at 600 K while
reacted with oxygen (O$_2$) and then investigated with helium low energy ion
scattering (LEIS). LEIS spectra collected at different scattering angles
confirm that oxygen does not adsorb to graphene, but instead intercalates
between the graphene and the substrate. The intercalated O$_2$ desorbs when the
sample is annealed to 800 K. It is shown that this is a much lower temperature
than is needed to remove chemisorbed atomic oxygen from Ru, thus inferring that
the intercalated oxygen is molecular. During the desorption process, some of
the graphene is etched away via a chemical reaction with the oxygen, with the
proportion desorbing as O$_2$ or reacting to etch the graphene being dependent
on the amount of intercalated O$_2$.
