Vibrational energy relaxation of azulene studied by the transient grating method. I. Supercritical fluids

The vibrational energy dissipation process of the ground-state azulene in supercritical xenon, carbon dioxide, and ethane has been studied by the transient grating spectroscopy. In this method, azulene in these fluids was photoexcited by two counterpropagating subpicosecond laser pulses at 570 nm, which created a sinusoidal pattern of vibrationally hot ground-state azulene inside the fluids. The photoacoustic signal produced by the temperature rise of the solvent due to the vibrational energy relaxation of azulene was monitored by the diffraction of a probe pulse. The temperature-rise time constants of the solvents were determined at 383 and 298 K from 0.7 to 2.4 in rho(r), where rho(r) is the reduced density by the critical density of the fluids, by the fitting of the acoustic signal based on a theoretical model equation. In xenon, the temperature-rise time constant was almost similar to the vibrational energy-relaxation time constant of the photoexcited solute determined by the transient absorption measurement [D. Schwarzer, J. Troe, M. Votsmeier, and M. Zerezke, J. Chem. Phys. 105, 3121 (1996)] at the same reduced density irrespective of the solvent temperature. On the other hand, the temperature-rise time constants in ethane were larger than the vibrational energy-relaxation time constants by a factor of about 2. In carbon dioxide, the difference was small. From these results, the larger time constants of the solvent temperature rise than those of the vibrational energy relaxation in ethane and carbon dioxide were interpreted in terms of the vibrational-vibrational (V-V) energy transfer between azulene and solvent molecules and the vibrational-translational (V-T) energy transfer between solvent molecules. The contribution of the V-V energy transfer process against the V-T energy transfer process has been discussed.