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. (C) 2004 Elsevier B.V. All rights reserved.