Properties and dynamics of the image potential states on graphite investigated by multiphoton photoemission spectroscopy

Abstract

Multiphoton photoemission spectroscopy of graphite using 267 nm (4.65 eV) and 400 nm (3.1 eV) excitation wavelength reveals spectroscopic features that allow the identification of the multiphoton excitation process and that correspond to the known bulk band structure. In addition, the $n=1$ and $n=2$ image potential states on graphite are identified, with binding energies of 0.85 and 0.15 eV, respectively. They are characterized by a vanishing quantum defect and are located close to the top of the band gap in the projected bulk band structure. Accordingly, the $n=1$ image potential state and the minimum of the interlayer band are both located about 4 eV above the Fermi level. This settles the ambiguities in the interpretation of the unoccupied band structure of graphite with respect to the energetic location of the interlayer band. Time-resolved two-photon photoemission spectroscopy yields a lifetime of $40\ifmmode\pm\else\textpm\fi{}6\mathrm{fs}$ for the $n=1$ image potential state. This rather long lifetime of an image potential state at the top of the band gap and the vanishing quantum defect are attributed to the two-dimensional structure of graphite.