Real-time vibrational mode-coupling associated with ultrafast geometrical relaxation in polydiacetylene induced by sub-5-fs pulses

The relaxation time of the free-exciton→geometrically relaxed 2 1Ag state in polydiacetylene is determined as 65–80 fs, depending on the probe energy. The vibrational non-equilibrium in the relaxed 2 1Ag state is characterized by the modulations of instantaneous frequencies and amplitudes of CC and C–C stretching modes with a 145-fs oscillation period. The frequency modulation is anti-phase between the two modes, and the amplitude and frequency are also anti-phase for both modes. These are explained by the coupling of the stretching modes through an in-plane bending mode with 230 cm−1. This is the first real-time observation of dynamic vibrational mode coupling.