Chirped modulation of molecular vibration in quinoidal thiophene after sub-5 fs excitation

Fourier transformation of the real-time trace of sub-5 fs pump-induced molecular vibration in quinoidal thiophene in solution revealed coupled modes to the electronic excitation at 206, 447, and 1466 cm−1. Spectrogram analysis of 1466 cm−1 (CC stretching) mode showed that the amplitude and frequency of the mode are modulated with another low-frequency mode at about 220 cm−1, which is down chirped being induced by the coupling with modes of even lower frequency, resulting in the irreversible relaxation due to high mode density in the low-frequency range. This Letter thus reports on observation of the modulation of vibrational frequency with a chirped frequency.