Unoccupied electronic states in polythiophene as probed by XAS and RAS

Abstract

AbstractPolythiophene is a typical conducting polymer. Unoccupied electronic states near the Fermi level of polythiophene polycrystalline powder were probed using S 1s X‐ray absorption spectroscopy (XAS) and S KL2, 3L2, 3 resonant Auger spectroscopy (RAS). Its overall XAS spectral shape resembles those of 2,‐2′‐bithiophene and α‐terthiophene polycrystalline powder. No energy shifts were observed in the π* and σ*(SC) states. In contrast, it was observed that the position of σ*(CC) resonance slightly shifts to lower energy concomitantly with the increase of thiophene repeating units. The normal Auger yield spectrum shows the lack of a delocalized peak, which suggests π*‐electrons remain localized at each thiophene unit during the S 1s core‐hole lifetime of ca 1.1 femtoseconds. Although π* and σ*(SC) states in XAS are not resolved because of their overlap of electronic transitions, they were separated by plotting spectator Auger yields as a function of excitation energy. Such unresolved states in XAS are demonstrated to be distinguishable using RAS technique if the Auger spectator shifts of the corresponding states differ mutually. Results of this study underscore that excitation energy‐dependent RAS can overcome the limitation of conventional XAS. Copyright © 2008 John Wiley & Sons, Ltd.