COLLISIONAL DEACTIVATION OF VIBRATIONALLY HIGHLY EXCITED AZULENE IN COMPRESSED LIQUIDS AND SUPERCRITICAL FLUIDS

The collisional deactivation of vibrationally highly excited azulene w as studied from the gas to the. compressed liquid phase. Employing sup ercritical 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 en ergy of 18 000 cm(-1) was generated by laser excitation into the S-1 a nd internal conversion to the S-0-ground state. The subsequent loss o f vibrational energy was monitored by transient absorption at the red edge of the S-3<--S-0 absorption band near 290 nm. Transient signals w ere converted into energy-time profiles using hot band absorption coef ficients from shock wave experiments for calibration and accounting fo r 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 ex periments. For Xe, CO2, and C2H6, the linear relation between cooling rate and diffusion coefficient scaled collision frequencies Z(D) turne d 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 f inite lifetime of collision complexes. (C) 1996 American Institute of Physics.