Femtosecond electron dynamics at the benzene/Ag(111) interface

The layer dependent evolution of the benzene/Ag(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 be nzene multilayer photoelectron kinetic energy spectra, all excitations poss ess free electron-like dispersions parallel to the Ag(111) surface, consist ent with image electronic states and not electron affinity levels in benzen e molecular crystals. The non-dispersive peak in the benzene multilayer spe ctra is assigned to be an n = 1 image state localized by structural disorde r. The binding energy and lifetime of the n = 1 image state has been measur ed for 1 to 5 layers of benzene. Adsorption of a benzene monolayer moves th e n = 1 image state electron closer to the metal, raising the n = 1 binding energy from - 0.77 eV to -0.84 eV and lowering the n = 1 lifetime from 36 to 20 femtoseconds. Adsorption of a bilayer lowers the n = 1 binding energy to -0.68 eV and raises the n = 1 lifetime to 45 fs. The lifetime and bindi ng energy remain constant from a bilayer to 5 layers of benzene. A dielectr ic continuum model successfully reproduces these trends in n = 1 binding en ergy and lifetime. Analysis of the model potential and the calculated wavef unctions shows the layer independence of the multilayer n = 1 lifetimes and binding energies to result from the trapping of the image state electron i n the screened image potential within the benzene layer. (C) 2000 Elsevier Science B.V. All rights reserved.