Combined photoelectron and metastable atom electron spectroscopy study of n-doped oligophenylene thin films

We have used ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) to study the changes in the valence electronic structure of oligophenylenes upon the deposition of alkali metals. We have fabricated thin films of p-quarterphenyl (4P) and p-sexiphenyl (6P) by thermal evaporation under ultrahigh vacuum conditions, resulting in partly oriented organic films. Subsequently, increasing amounts of alkali metals (Na, K, Rb, Cs in individual experiments) were deposited onto those in a stepwise manner. After each step, UPS and MAES spectra were recorded. We find new emissions within the formerly empty energy-gap of the organic materials induced by the charge transfer from the alkali metals. These are attributed to the formation of bipolarons. We find that the deexcitation probability of He∗ (21S) atoms on the doped organic surfaces exceeds that of He∗ (23S) by up to four times.