Comprehensive measurement of the S1 azulene relaxation dynamics and observation of vibrational wavepacket motion

The vibrational excess energy dependence of the internal conversion of S1 azulene in cyclohexane was measured with 50 fs resolution. The decay time decreases from 1.7 ps for the vibrationless S1 state to 400 fs at 1300 cm−1. An upper limit of 2300 cm−1 is deduced for the energetic location of the conical intersection to the S0 state responsible for the ultrafast decay. Further increased excess energy causes biexponential decay without significant decrease in the decay times up to 5500 cm−1. Strong oscillatory signal contributions are found due to low-frequency wavepacket motion not directly excited by the optical transition.