The orientation of CO intercalated between graphene and Ru(0001)

$^{13}$CO molecules are intercalated under a single layer graphene film on Ru(0001) and interrogated with helium low energy ion scattering. Single scattering is used to determine the mass distribution of atomic species visible to the ion beam and detector, and the scattering angle is varied to distinguish adsorbed from intercalated molecules. At room temperature, CO intercalates as molecules that sit upright with the O end on top, as on clean Ru. The intercalated CO tilts, more than it does on clean Ru, when the temperature is raised. This is presumably due to increased vibrational amplitudes combined with the confining effect of the graphene film.