The formation and evolution of the gap states formed with increasing C
s concentration in thin sexiphenyl (6P) films is followed with ultravi
olet photoemission spectroscopy (UPS). Two distinct doping regimes are
apparent, experimental considerations suggest that in the lower-dopin
g regime bipolaron-like (dianionic 6P2-) states are formed. In the hig
her-doping regime, approaching Cs saturation, a 0.4 eV splitting in th
e gap states is observed which is attributed to interacting bipolaron
states. VEH calculations for 6P2- and 6P4- are in very good agreement
with the experimental spectra and lend strong support to the notion th
at a bipolaron to intrachain interacting bipolaron transition occurs w
ith increasing Cs concentration.