Radiation induced degradation and surface charging of organic thin films in ultraviolet photoemission spectroscopy

The effect of vacuum ultraviolet radiation on the: valence electronic structure of the electroluminescent organic materials p-sexiphenyl (6P) and tris-(8-hydroxy quinoline) aluminum (Alq(3)) was investigated by ultraviolet photoemission spectroscopy (UPS). The intense radiation of an undulator at the storage ring BESSY II (Berlin) caused a loss of conjugation in 6P, evidenced by a decrease in intensity of delocalized pi -orbitals in the UPS spectra. Depending on the degree of film degradation, surface charging was observed for both materials. It is shown that by illuminating the sample with laser light that can be absorbed by the organic/metal substrate system the surface charging could be compensated. Thus, the generation of free charge carriers by optical means appears to be a useful substitute for the use of an electron flood-gun in photoemission experiments, whenever sensitive samples could suffer from irradiation with electrons. (C) 2001 Elsevier Science B.V. Ah rights reserved.