Femtosecond electron dynamics at the benzenerAg 111 interface

Ž . The layer dependent evolution of the benzenerAg 111 unoccupied electronic structure and electron dynamics have been investigated with time and angle resolved two photon photoemission. With the exception of one peak in the benzene Ž . multilayer photoelectron kinetic energy spectra, all excitations possess free electron-like dispersions parallel to the Ag 111 surface, consistent with image electronic states and not electron affinity levels in benzene molecular crystals. The non-dispersive peak in the benzene multilayer spectra is assigned to be an ns1 image state localized by structural disorder. The binding energy and lifetime of the ns1 image state has been measured for 1 to 5 layers of benzene. Adsorption of a benzene monolayer moves the ns1 image state electron closer to the metal, raising the ns1 binding energy from y0.77 eV to y0.84 eV and lowering the ns1 lifetime from 36 to 20 femtoseconds. Adsorption of a bilayer lowers the ns1 binding energy to y0.68 eV and raises the ns1 lifetime to 45 fs. The lifetime and binding energy remain constant from a bilayer to 5 layers of benzene. A dielectric continuum model successfully reproduces these trends in ns1 binding energy and lifetime. Analysis of the model potential and the calculated wavefunctions shows the layer independence of the multilayer ns1 lifetimes and binding energies to result from the trapping of the image state electron in the screened image potential within the benzene layer. q 2000 Elsevier Science B.V. All rights reserved.