Photoluminescence spectra with high vibronic resolution were measured for highly ordered thin (25 Å and 32 Å) films of the linear π-conjugated oligomer α-quaterthiophene on highly oriented pyrolytic graphite and Ag(111). At low temperatures (1.3–40 K), we observe systematic changes in the line positions $\left(\sim 50{\mathrm{cm}}^{-1}\right)$ and line shapes for the vibronic side bands, but only an intensity decrease for the pure electronic $S_0\leftarrow S_1$ transition. This reveals the formation of an $S_1$ exciton band with a dispersion in $k$ space due to intermolecular resonance coupling. The nonlocal character of the excitonic state is expected to be relevant also for other linear π-conjugated systems.

• Received 29 May 2003

DOI:https://doi.org/10.1103/PhysRevB.69.125420