PREFERENTIAL SOLVATION IN THE COLLISIONAL DEACTIVATION OF VIBRATIONALLY HIGHLY EXCITED AZULENE IN SUPERCRITICAL XENON ETHANE MIXTURES/

The collisional deactivation of vibrationally highly excited azulene w as studied in equimolar supercritical mixtures of xenon and ethane at 385 K from gas-to liquid-phase densities. Azulene with an energy of si milar to 20 000 cm(-1) was generated by laser excitation into the S-1 state and subsequent internal conversion to the S-0 ground state. The loss of vibrational energy was monitored by transient absorption at t he red edge of the S-3 <-- S-0 absorption band at 290 nm. Transient si gnals were converted into energy-time profiles using hot band absorpti on coefficients from shock wave experiments for calibration and accoun ting for solvent shifts of the spectra. Under all conditions, the ener gy decays were exponential. At densities below 1 mol/L, the observed c ollisional deactivation rate constants k(c) of the mixture were equal to the sum of the individual contributions of ethane and xenon collisi ons as expected from simple gas kinetics. At mixture densities above 2 mol/L, k(c) is smaller than the deactivation rate constant found in n eat ethane at half the density. This behavior can be rationalized by a n isolated binary collision (IBC) model which relates the collision fr equency Z to the radial distribution function g(r) of an attractive ha rd-sphere particle in a Lennard-Jones fluid. Radial distribution funct ions obtained by Monte Carlo simulations clearly show that at high den sities the less efficient collider xenon preferentially solvates the a zulene molecule, reducing the number of azulene-ethane collisions and, therefore, the overall collisional deactivation rate constant with re spect to neat ethane solutions.