Ultrafast relaxation in conjugated polymers

We have developed Raman gain spectroscopy with femtosecond resolution. The advantage of the method is as follows: (1) There is no disturbance from fluorescence as in spontaneous Raman scattering; (2) The interference of the signal with nonresonant background does not appear. This is extremely advantageous to time-resolved CARS spectroscopy. We have applied this new method to excitons in polydiacetylene (PDA), polythiophenes, and a polyacetylene derivative with only 1.5 ps lifetime. This produces vibrational spectra with the highest time resolution ever reported. Time-resolved resonance Raman spectroscopy has been recognized as a powerful method for studying structures of transient species and electronic excited states. By applying the new methods to a polydiacetylene (PDA) crystal epitaxially grown on a KCl substrate, we could observe Raman spectrum of the exciton state in PDA with only 1.5 ps lifetime.