Influence of Ar, Kr, and Xe layers on the energies and lifetimes of image-potential states on Cu(100)

The influence of well-ordered adlayers of Ar, Kr, and Xe on the energetic and dynamical properties of image-potential states on Cu(100) has been investigated in a comprehensive study using time-resolved two-photon photoemission (2PPE). The effect of these insulating films varies systematically with the electron affinity EA of the condensed rare gases and with the film thickness. For the electron-repulsive Ar layers (EA = -0.25 eV), a strong lifetime increase of the n = 1 state from 40 fs on clean Cu(100) to as much as 10 ps at a coverage of 5 monolayers is observed. For Kr and Xe layers (EA = +0.3 and +0.5 eV, respectively), decoupling from the metal is less efficient. These layers exhibit quantum-well-like resonances of the n = 2 state as a function of layer thickness. The energies of the series of states depend characteristically on the affinity level and the dielectric constant of the films. A microscopic model is developed that includes the discrete atomic structure of the adsorbate layers. It is capable of describing the experimental results to a high degree of quantitative agreement.