Low-Lying Unoccupied Electronic States in Graphene on Ir(111) and Intercalated Interfaces by Time- and Angle-Resolved Two-Photon-Photoemission*

We report experimental and theoretical work to understand how intercalation modulates unoccupied electronic states in a metal/graphene interface. We use time- and angle-resolved two-photon-photoemission spectroscopy to map the evolution of the electronic band structure and record the electron dynamics of an epitaxially grown graphene on Ir(111) as it undergoes through a cycle involving oxygen intercalation and deintercalation. Oxygen intercalation is carried out in situ and LEED is used to determine the crystallinity of the interface in the presence or absence of the intercalated adatoms. The image potential state and its electron dynamics are examined by both mono- and bi-chromatic 2PPE. The observation of an oxygen induced state is discussed and explained by DFT calculations. Furthermore, a recently proposed effective potential model at the graphene/metal interface is developed to accommodate intercalation and interpret our experimental electronic structure variation with good agreement with the model.