Bithiophene is studied with varying Cs dosing as a model for the n-dop
ing of thiophene-based conducting polymers. The evolution of the elect
ronic structure with increasing Cs concentration is followed both indi
rectly, by electron-energy-loss spectroscopy (EELS), and directly by u
ltraviolet-photoemission spectroscopy (UPS). In both spectroscopies th
e behavior of the states formed in the gap indicates two doping regime
s. The EELS results parallel the optical-absorption results for the p-
doping of longer thiophene oligomers and polythiophene, and suggest a
polaron (anion) to bipolaronlike (dianion) transformation. The UPS res
ults are consistent with this interpretation, showing first in the low
-doping regime the appearance of two states in the band gap which move
closer together in the higher-doping regime. This constitutes direct
observation of a transition from polaron to bipolaron states, albeit i
n a model system. Consideration of the photoionization cross sections
indicates that the deeper (bi)polaron level has C 2p character while t
he shallow level, filled by donation from the Cs, has significant S 3p
character.