Observation of electron two-dimensional standing-wave patterns at the surface of an insulating layer by scanning tunneling microscopy

Electron standing wave patterns around single hexa-peri-hexabenzocoronene (HBC) molecules adsorbed on the surface of thin KBr(001) films grown on Ag(111) are observed by scanning tunneling microscopy. We suggest that they result from the scattering, by the molecules, of the electrons populating the surface resonance associated with the lowest image potential state of the insulating film on a metal system. These patterns are nearly independent on the thickness of the film from two to five KBr monolayers, implying that they are localized near the surface of the insulating film. The electrons exhibit a parabolic dispersion relation which yields an effective mass m* = 0.62 +/- 0.2 m(e).