The influence of well-ordered adlayers of Argon on binding energies and inelastic lifetimes of imagepotential states and resonances on Cu͑111͒ and Ag͑111͒ 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 =1͒ image-potential state above the minimum of unoccupied projected bulk bands on Cu͑111͒ 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 Ag͑111͒ 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 ͑ML͒. On Cu͑111͒ the exponential increase is considerably reduced when the n = 1 state becomes a resonance. Up to 10 ML of Ar the lifetime on Cu͑111͒ 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.

The dependence of the inelastic lifetime of electrons in the first n = 1 image-potential state of clean and rare-gas covered Ag(111), Cu(111), and Cu(100) surfaces on their momentum parallel to the surface has been studied experimentally by means of time-and angle-resolved two-photon photoemission spectroscopy (2PPE) and theoretically by calculations based on the many-body theory within the self-energy formalism. Similar to the previously studied clean Cu(100) surface, the theoretical results are in excellent agreement with the experiment findings for Cu(111). For Ag(111), the theory overestimates the decay rate and its momentum dependence, which is attributed to the neglect of surface plasmon excitations. With increasing parallel momentum, the n = 1 state shifts out of the projected bulk band gap on both surfaces and turns into an image-potential resonance. This opens an additional decay channel by resonant electron transfer into the bulk, which is theoretically treated by the application of the wave packet propagation approach. The expected stronger increase of the decay rate upon crossing the edge of the band gap, however, is not observed in the experiment. The decoupling of the image-potential states from the metal surface upon adsorption of rare-gas layers results in a decrease of the decay rate as well as of its momentum dependence by a similar factor, which can be successfully explained by the change of interband and intraband contributions to the total decay rate.

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