Effect of electrostatic screening on apparent shifts in photoemission spectra near metal/organic interfaces

Photoemission spectra of very thin organic films differ from that of thicker bulklike films. In particular, the binding-energy shifts in molecular orbitals at metal/organic interfaces vary as a function of the organic over-layer thickness. Using a simple image-charge model it is found that electrostatic screening can have a significant effect on the final-state relaxation energy. Good agreement between experimental results and theoretical calculations for a variety of dielectric substrates indicate that reported thickness dependent energy-level shifts in organic overlayer spectra can be accounted for by electrostatic screening. Models of organic interfaces based on photoemission spectra of organic thin films should therefore be re-examined.