News February 2012 Time-Resolved Two-Photon Photoemission of Unoccupied Electronic States of Periodically Rippled Graphene on Ru(0001) The unoccupied electronic states of epitaxially grown graphene on Ru(0001) have been explored by time- and angle-resolved two-photon photoemission (2PPE). We identify a Ru derived resonance and a Ru/graphene interface state at 0.91 eV and 2.58 eV above the Fermi level, as well as three image-potential derived states close to the vacuum level. The most strongly bound, short lived and least dispersing image-potential state is suggested to have some quantum-well character with a large amplitude below the graphene hills. The two other image-potential states are attributed to a series of slightly decoupled states. Their lifetimes and dispersions are indicative of electrons moving almost freely above the valley areas of the moiré superstructure of graphene. N. Armbrust, J. Güdde, P. Jakob, and U. Höfer, Phys. Rev. Lett. 108, 056801 (2012).

December 2011 PRL Editors' Suggestions: Trapping of image-potential resonances on free-electron-like metal surface Image-potential states have been studied by two-photon photoemission (2PPE) for the surface of Al(100) where the whole series is energetically degenerate with free-electron-like bulk states. In contrast to expectations, the series of resonances is not smeared out to one broad structure as a result of a strong coupling to the bulk continuum. Instead, the first resonance (n=1) is found to be suppressed and the resonances with quantum numbers n=2,3,4,5 are resolved as individual peaks in the time-resolved spectra. Both effects are suggested to be a consequence of resonance trapping. M. Winter, E. V. Chulkov, and U. Höfer, Phys. Rev. Lett. 107, 236801 (2011).

Home

Page updated 9 February 2012 . . . Webmaster