Real-time observation of vibrational coherence persisting after internal conversion and vibrational relaxation in cyanine dye molecules

Vibrational coherence after internal conversion from the higher-lying excited singlet state to the first excited singlet state (Sn→S1) and vibrational relaxation in cyanine dye molecules in solution are real-time observed with two-color pump–probe experiments with 20-fs time resolution. The intensity of oscillating signal due to 300-cm−1 molecular vibration increases with time constant of about 1 ps. This phenomenon shows that the vibrational coherence preserves even after a large energy loss, 1.55 eV, by internal conversion and vibrational relaxation.