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

Abstract

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 benzene 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 n=1 image state localized by structural disorder. The binding energy and lifetime of the n=1 image state has been measured for 1 to 5 layers of benzene. Adsorption of a benzene monolayer moves the 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 binding energy remain constant from a bilayer to 5 layers of benzene. A dielectric continuum model successfully reproduces these trends in n=1 binding energy and lifetime. Analysis of the model potential and the calculated wavefunctions shows the layer independence of the multilayer n=1 lifetimes and binding energies to result from the trapping of the image state electron in the screened image potential within the benzene layer.