Abstract
Electric field induced changes of the absorption spectra are used to study the \ensuremath{\pi} electron states of isolated 4BCMU and 3BCMU polydiacetylene (PDA) chains. The strong excitonic transition, common to all PDA chains, responds to moderate electric fields according to a quadratic Stark effect. Weak transitions below this exciton respond identically. The polarizability of all these excitons is the same and equal to that in fully polymerized PDA's. About 580 meV higher in energy an additional response broadening with increasing field is observed whose amplitude increases sublinearly with field. This behavior is in perfect agreement with what is expected for the Franz-Keldysh effect of continuum states, yielding a reduced mass of only 0.05${\mathit{m}}_{0}$. This small mass is consistent with an exciton size of 5 repeat units, estimated from the Stark shift of the excitons. The spectral range of the well resolved Franz-Keldysh oscillations points to a coherence length of \ensuremath{\pi} states of 200-400 \AA{}. \textcopyright{} 1996 The American Physical Society.
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