Observation of a Fano resonance in photoemission

We report on the observation of a Fano resonance in normal emission ultravi olet photoelectron spectra of Cu or Ag impurities embedded in a Xe monolaye r on highly oriented pyrolytic graphite. We show that Brillouin-zone foldin g resulting from the relatively weak perturbation by the (root 3 x root 3)R 30 degrees commensurate Xe overlayer couples K-point electrons with Gamma-p oint electrons of pure graphite. Interference between this continuum of sta tes and the discrete Cu 4s or Ag 5s states in the initial state leads to a typical Fano line profile in the spectra. This finding provides a new and p romising avenue for the detection and characterization of impurities or sma ll clusters at surfaces. [S0031-9007(99)08842-0].