Vibrational phase characterization in femtosecond-pumped molecules by path-length modulation pump–probe spectroscopy

Highly sensitive dynamics of molecular vibration in a cyanine dye, 1,1′,3,3,3′,3′-hexamethyl-4,4′,5,5′-dibenzo-2,2′-indotricarbocyanine (HDITC), was studied by path-length modulation pump–probe spectroscopy using 20 fs pulses. The observed real-time spectroscopic signal was decomposed into seven vibrational modes with well-determined phases by the singular-value decomposition method. Dependence of the vibrational phases for these modes on the probe wavelength was analyzed by the effective linear response function method. A significant change observed in this dependence by the relation between the excitation pulse duration and the mode frequency is discussed in terms of the wavepacket formation scheme.