Spectroscopy of image-potential states with inverse photoemission

The binding energies of image-potential states are studied in a systematic way for metals [Cu(111), Cu(100), Cu(110), Ag(111), Au(111), and Au(100)], a semimetal [Sb(100)], and a layered compound (1T-${\mathrm{TiS}}_2$). Data for Au(111) show that image-potential states exist even in the presence of bulk states. The coupling with bulk states is weak as indicated by the narrow (≤0.1 eV) width of the image states. The binding energies of the lowest state cluster around 0.7 eV for a variety of metal and semimetal surfaces indicating a universal phenomenon. This value is close to the hydrogenic binding energy of (1/16) Ry ∼0.85 eV for the n=1 state, which indicates that the simple Coulombic image potential dominates the energy balance.