Two-dimensional localization of adsorbate/substrate charge-transfer excited states of molecules adsorbed on metal surfaces

Electron energy loss experiments provide evidence for the two-dimensional s patial localization of the metal/molecule charge-transfer excited state of pyromellitic dianhydride adsorbed on copper single-crystal surfaces in ultr ahigh vacuum. Codosing the surface with atomic cesium splits the interfacia l loss peak into a doublet, whose energies are independent of cesium covera ge. This behavior is in contrast to the smooth, continuous shift in energy expected from the classical picture of alkali metal adsorption in which the substrate Fermi energy increases with coverage at low coverage. Line shape analysis suggests that the localized region includes the four adsorbed mol ecules nearest each cesium adatom, (C) 1999 American Institute of Physics. [S0021-9606(99)70101-7].