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

The complete electronic (occupied and unoccupied states) and vibrational st ructure of thin films of bithiophene in undoped form and after alkali dopin g have been investigated experimentally and theoretically. The ultraviolet (UV) photoemission and inverse photoemission spectra demonstrate clearly th e filling of the LUMO of bithiophene in the doping process due to charge tr ansfer from the alkali atoms, and density functional theory (DFT) calculati ons confirm the formation of dianion (bipolaronlike) states in the HOMO-LUM O band gap at high doping levels, respectively. High-resolution electron en ergy loss spectra (HREELS) have been used to probe the vibrational structur e of the bithiophene films, and the formation of a quinoidic structure on C s doping is well reflected in HREELS by the advent of an intense vibrationa l 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. [S0021-9 606(99)50616-8].