Optical frequency- and vibrational time-resolved two-dimensional spectroscopy by real-time impulsive resonant coherent Raman scattering in polydiacetylene

Real-time impulsive resonant Raman scattering induced by a 4-fs ultrashort laser pulse was spectrally resolved with a multichannel lock-in amplifier. An optical and vibrational two-dimensional spectrum was introduced to separate the effects of electronic and vibrational broadening in the power spectrum of the vibrations. Spectrally resolved real-time oscillations can identify the wave packet motion associated with the stretching vibrational mode of the carbon-carbon double and triple bonds of the polymer polydiacetylene. Calculated nonlinear polarizations which take into account the ground-state wave packets reproduced the observed dynamics fairly well. It is confirmed that the ground-state vibrations play a significant role in the dynamics of the wave packet even far below the absorption edge, as well as the excited-state vibrations.