Electronic Band and Orbital Properties of Cs-Doped Pentacene Thin Films

We studied the electronic band structure of Cs-doped pentacene films grown on Cu(119). The Cs doping of pentacene films grown on Cu(119) determines relevant changes in the band structure. Angle-resolved photoemission spectroscopy reveals a formation of a new electronic state at 0.6-0.7 eV below the Fermi energy. This state results from the filling of the lowest unoccupied molecular orbital as also shown by X-ray absorption at C 1s edge. At doping level of 1.8 Cs atom per molecule it disperses, forming an electronic band of 130 meV width along the direction perpendicular to the molecular plane, while no dispersion is observed in plane. Our results indicate that the anisotropic charge delocalization mediated by Cs atoms along the molecular stacking direction takes place in doped pentacene films.