Electronic and vibrational structure of thin films of bithiophene:: Undoped and alkali-doped states

The complete electronic (occupied and unoccupied states) and vibrational structure of thin films of bithiophene in undoped form and after alkali doping have been investigated experimentally and theoretically. The ultraviolet (UV) photoemission and inverse photoemission spectra demonstrate clearly the filling of the LUMO of bithiophene in the doping process due to charge transfer from the alkali atoms, and density functional theory (DFT) calculations confirm the formation of dianion (bipolaronlike) states in the HOMO-LUMO band gap at high doping levels, respectively. High-resolution electron energy loss spectra (HREELS) have been used to probe the vibrational structure of the bithiophene films, and the formation of a quinoidic structure on Cs doping is well reflected in HREELS by the advent of an intense vibrational structure at 206 meV, which is due to the stiffened C-C interring bond as also predicted by theory. (C) 1999 American Institute of Physics. [S0021-9606(99)50616-8].