Measuring the electronic corrugation at the metal/organic interface

Angle-resolved two-photon photoemission is used to probe the image potential states on monolayers of metal-free phthalocyanine (H2Pc) and cobalt phthalocyanine (CoPc) on the Ag(100) and Ag(111) surfaces. We find that in the plane of the surface the normally flat potential landscape becomes significantly corrugated in the presence of the phthalocyanine lattice, causing the opening of a band gap in the first image potential state, in agreement with previous findings. Interestingly, the data show that the absence of the electron dense metal center does not qualitatively alter this picture. Experiments conducted on Ag(100) and Ag(111) crystals demonstrate that the surface band structure and symmetry of the metal surface plays a minor role in the band folding phenomenon. A two-dimensional model that takes into account both the band structure of the substrate and the corrugation of the potential landscape in the surface plane due to the molecular lattice is presented and compared to experiment. This model enables the observation that, counter to intuition, the Co metal center at the CoPc/Ag interface actually smooths the interfacial potential relative to that of the H2Pc/Ag interface. We suggest that the strong corrugation of the potential at the organic/metal interface measured herein may account for the recent observation of surface umklapp scattering in recent ultraviolet photoemission experiments on organic/metal interfaces.