WANNIER EXCITONS AND FRANZ-KELDYSH EFFECT OF POLYDIACETYLENE CHAINS DILUTED IN THEIR SINGLE-CRYSTAL MONOMER MATRIX

Electric field induced changes of the absorption spectra are used to s tudy the pi electron states of isolated 4BCMU and 3BCMU polydiacetylen e (PDA) chains. The strong excitonic transition, common to all PDA cha ins, responds to moderate electric fields according to a quadratic Sta rk effect. Weak transitions below this exciton respond identically. Th e polarizability of all these excitons is the same and equal to that i n fully polymerized PDA's. About 580 meV higher in energy an additiona l response broadening with increasing field is observed whose amplitud e increases sublinearly with field. This behavior is in perfect agreem ent with what is expected for the Franz-Keldysh effect of continuum st ates, yielding a reduced mass of only 0.05m(0). This small mass is con sistent with an exciton size of 5 repeat units, estimated from the Sta rk shift of the excitons. The spectral range of the well resolved Fran z-Keldysh oscillations points to a coherence length of pi states of 29 0-400 Angstrom.