Vibrational dephasing of the -NCN- anti-symmetric stretching mode of carbodiimide studied by infrared photon echo method

We have studied vibrational dephasing of the carbodiimide (-NCN-) anti-symmetric stretching mode of di-cyclohexylcarbodiimide (DCCD) in carbon tetrachloride and N,N-dimethylformamide by infrared photon echo technique. Both time-integrated and frequency-resolved photon echo measurements have been carried out. The time-integrated signal shows a nonexponential decay with a time scale of a few picoseconds. The signal shows its peak at around the origin of the delay time, suggesting that the vibrational frequency fluctuation is in the fast modulation limit. We found unusual frequency dependence in the rise of the frequency-resolved photon echo signals. Simulations of the photon echo signal based on the conventional formalism of nonlinear infrared spectroscopy cannot reproduce the dependence. The origin of the unusual behaviors is discussed.