Observation of a Fano resonance in photoemission

We report on the observation of a Fano resonance in normal emission ultraviolet photoelectron spectra of Cu or Ag impurities embedded in a Xe monolayer on highly oriented pyrolytic graphite. We show that Brillouin-zone folding resulting from the relatively weak perturbation by the (root 3 x root 3)R30 degrees commensurate Xe overlayer couples K-point electrons with Gamma-point electrons of pure graphite. Interference between this continuum of states 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 promising avenue for the detection and characterization of impurities or small clusters at surfaces. [S0031-9007(99)08842-0].