Excitation-induced modulation of vibrational frequency and amplitude in dye molecules was studied using sub-5-fs pulses. The frequency and amplitude modulations of the ring-breathing mode were clearly observed, indicating that the mode is coupled to another vibrational mode. The common modulated frequency and the depth of phase modulations were determined from the instantaneous frequency and amplitude. The two-dimensional (2D) potential surfaces were determined using a simple model of two-mode coupling. The anharmonicity appearing in the potential surfaces represents the Duschinsky effect.