Several vibrational modes impulsively excited by 6.7 fs pulses were observed in a quinoid thiophene. From the amplitude decay time and the phase of vibration, the modulation mechanism of the electronic transition by the wave packet motion is discussed and classified in terms of vibronic coupling. Modes with wavenumbers of 1343 and 1420 cm(-1) are attributed to the wave packet motions on the potential surface of the excited and ground state, respectively, and a mode with 1539 cm(-1) is due to those of both states. The apparent low-frequency modulation of these three modes is ascribed to the artificial interference effect in the calculation of the spectrogram. (C) 2007 Elsevier B.V. All rights reserved.