Collisional deactivation of vibrationally highly excited azulene in compressed liquids and supercritical fluids

The collisional deactivation of vibrationally highly excited azulene was studied from the gas to the. compressed liquid phase. Employing supercritical fluids like He, Xe, CO2, and ethane at pressures of 6-4000 bar and temperatures greater than or equal to 380 K, measurements over the complete gas-liquid transition were performed. Azulene with an energy of 18 000 cm(-1) was generated by laser excitation into the S-1 and internal conversion to the S-0*-ground state. The subsequent loss of vibrational energy was monitored by transient absorption at the red edge of the S-3<--S-0 absorption band near 290 nm. Transient signals were converted into energy-time profiles using hot band absorption coefficients from shock wave experiments for calibration and accounting for solvent shifts of the spectra. Under all conditions, the decays were monoexponential. At densities below 1 mol/l, collisional deactivation rates increased linearly with fluid density. Average energies [Delta E] transferred per collision agreed with data from dilute gas phase experiments. For Xe, CO2, and C2H6, the linear relation between cooling rate and diffusion coefficient scaled collision frequencies Z(D) turned over to a much weaker dependence at Z(D)>0.3 ps(-1). Up to collision frequencies of Z(D)=15 ps(-1) this behavior can well be rationalized by a model employing an effective collision frequency related to the finite lifetime of collision complexes. (C) 1996 American Institute of Physics.