Sub-5-fs nonlinear optical processes in a polydiacetylene film

Abstract The relaxation time of free-exciton → geometrically relaxed 21Ag-state in a polydiacetylene is determined as 65–80 fs depending on the probe energy. It includes the processes of internal conversion, geometrical relaxation from the acetylene-type configuration to the butatriene configuration, and thermalization among intrachain vibrational modes. The vibrational nonequilibrium in the relaxed 21Ag- state is characterized by the modulations of instantaneous frequencies and amplitudes of C–C, CC, and CC stretching modes with long oscillation periods. Most prominent modulation was found between C–C and CC mode. This is the first real-time observation of dynamic vibrational mode coupling.