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

Multiphoton photoemission spectroscopy of graphite using 267 nm (4.65 eV) a nd 400 nm (3.1 eV) excitation wavelength reveals spectroscopic features tha t allow the identification of the multiphoton excitation process and that c orrespond to the known bulk band structure. In addition, the n=1 and n=2 im age potential states on graphite are identified, with binding energies of 0 .85 and 0.15 eV, respectively. They are characterized by a vanishing quantu m 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 min imum of the interlayer band are both located about 4 eV above the Fermi lev el. This settles the ambiguities in the interpretation of the unoccupied ba nd structure of graphite with respect to the energetic location of the inte rlayer band. Time-resolved two-photon photoemission spectroscopy yields a l ifetime of 40+/-6 fs for the n=1 image potential state. This rather long li fetime of an image potential state at the top of the band gap and the vanis hing quantum defect are attributed to the two-dimensional structure of grap hite. [S0163-1829(99)09547-8].