Vibrational energy relaxation of azulene in the S2 state. I. Solvent species dependence

We have measured the time-resolved fluorescence spectra of azulene in the $S_2$ state in compressed gases and in liquids. We have found that the band shape of the fluorescence changes significantly in the earlier time scale after the photoexcitation when large excess energy (about 6500 cm⁻¹) is given. The change of the band shape is similar both in the compressed gases and in the liquids, although the time scales of the change are quite different. We have measured the excitation energy dependence of the fluorescence band shape of the isolated molecule separately, and shown that the time dependence of the fluorescence band shape in gases and liquids corresponds to the vibrational energy relaxation in the $S_2$ state. Comparing with the excitation energy dependence of the fluorescence band shape of the isolated molecule, we have succeeded in determining the transient vibrational excess energy. The vibrational energy relaxation rates in the $S_2$ state are 1–2 times faster than those in the ground state both in compressed gases and in liquids.