Brillouin zone folding and Fano resonances in commensurate rare gas monolayers observed in photoemission

We report on the observation of Brillouin zone folding in normal emission ultraviolet photoelectron spectra of Ar, Kr, and Xe monolayers on highly oriented pyrolytic graphite. We show that the folding results from the relatively weak perturbation by the (3×3)R30° commensurate overlayers which couples K-point electrons with Γ-point electrons of pure graphite. Discrete Cu 4s or Ag 5s states of Cu or Ag impurities embedded in the rare gas monolayer interact with the continuum of the Brillouin zone-folded states in the initial state. As a consequence a typical Fano line profile in the valence photoemission spectra is observed. This finding provides a new and promising avenue for the detection and characterization of impurities or small clusters at surfaces.