Valence-band ultraviolet photoemission spectra of undoped poly(methylthiophene) show an extended $\pi$ bonding at 1.05 eV from the Fermi level, $E_F$. Doping with Cl${\mathrm{O}}_4^{-}$ proceeds through uniform extraction of $\pi$-bonding electrons resulting in (i) an extension of the $\pi$ band and (ii) an $E_F$ shift by 0.8 eV relative to this band and 0.5 eV relative to the core levels. Band-gap narrowing reduces the $\pi \to {\pi }^{*}$ shakeup energy of the x-ray photoemission spectroscopy Cl$s$ peak from 2.7 to 1.7 eV with a strong intensity enhancement.

• Received 3 October 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.1862