Investigation of polarization effects in organic thin films by surface core-level shifts

We study charge delocalization and polarization effects in organic thin films by comparing surface- and bulk-sensitive, high-resolution C 1s photoemission data using tunable synchrotron radiation. In three cases, perylene-tetracarboxylicacid-dianhydride, coronene, and metal-free phthalocyanine, no surface core level shifts are observed (upper limit 100 meV). This unexpected finding shows that polarization effects, and hence polarization differences between surface and bulk, are rather small, and implies that the effective charge is not localized on the core-ionized molecule but delocalized over several neighboring molecules.