Femtosecond time-resolved resonance Raman gain spectroscopy in polydiacetylene

Geometrical relaxation of the main chain configuration in polydiacetylene (PDA) has been investigated with time-resolved resonance Raman gain spectroscopy with 300-fs resolution. The Raman signal due to a self-trapped exciton in PDA has been observed at 1200 ${\mathrm{cm}}^{\mathrm{-}1}$. The observed Raman signals offer direct evidence of the structure of a self-trapped exciton being a butatrienelike structure after geometrical relaxation from the acetylenelike structure in the free exciton state.