Observation of the transition from image-potential states to resonances on argon-covered Cu(111) and Ag(111) by time-resolved two-photon photoemission

The influence of well-ordered adlayers of Argon on binding energies and inelastic lifetimes of image-potential states and resonances on Cu(111) and Ag(111) has been investigated by means of time-resolved two-photon photoemission spectroscopy. The adsorption of Ar layers on metals results in a strong decoupling of the image-potential states that goes along with an exponential increase in their inelastic lifetimes and a lowering of their binding energies. The latter shifts the first (n=1) image-potential state above the minimum of unoccupied projected bulk bands on Cu(111) and thereby induces a transition from an image-potential state to a resonance. This leads to a strikingly different dependence of the inelastic lifetime of the n=1 state on Ar layer thickness for the two surfaces. On Ag(111) the lifetime shows a continuous exponential increase with layer thickness from 32 fs on the clean surface to about 6 ps for an Ar coverage of four monolayers (ML). On Cu(111) the exponential increase is considerably reduced when the n=1 state becomes a resonance. Up to 10 ML of Ar the lifetime on Cu(111) does not exceed 3 ps. This rather unexpected behavior can neither be explained by a simple tunneling picture of the tunneling through thin Ar films nor by model calculations using a one-dimensional model potential that accounts for the most important electronic properties of both the metal substrate as well as of the Ar layers.