Franz-Keldysh effect of band states in polydiacetylene

Electroabsorption spectra of polydiacetylene single crystals with well ordered chains show 0.5eV above the excitonic absorption edge a strong signal from states which contribute little to absorption. While the strongly absorbing excitons respond to an external field by a quadratic Stark effect this particular signal is attributed to the Franz-Keldysh effect, the response of a free carrier band. The extended band states are very sensitive to an external field but also to disorder which destroys the coherence of free motion. Experimental spectra of the Franz-Keldysh effect of well ordered chains will be compared with those of semiconductors and with model calculations for a one-and a three-dimensional electron gas. These calculations reveal that one-dimensional bands respond much more sensitive to electric fields than three-dimensional band states. The comparison of the spectral lineshapes points to a forbidden parity state very close to the band edge which enhances some spectral features. The Franz-Keldysh effect deteriorates rapidly with disorder which is interpreted as due to localization of bandstates by disorder. The Stark effect of such near degenerate states localized by boundary conditions can lead to similar spectra as the Franz-Keldysh effect.