Surface-umklapp in ARPES at an organic-metal interface

We show that angle-resolved photoemission performed using low-energy photons on an organic-metal interface allows to clearly distinguish genuine interface states from features of substrate photoelectrons diffracted by the molecular lattice. As a model system an ordered monolayer of Zn-phthalocyanine is used as a
diffraction lattice to probe the electronic band structure of a Ag(110) substrate. Photoemission close to normal emission geometry reveals strongly dispersive features absent in the pristine substrate spectra. Density functional theory modeling helped identifying these as bulk sp direct transitions undergoing surface-umklapp processes. The present results establish the important role of final-state diffraction effects in photoemission experiments at organic-inorganic interfaces. Moreover, this phenomenon is illustrated through a series of examples of noble-metal single-crystal surfaces covered by monolayers of large π-conjugated organic molecules.