Collisional deactivation of vibrationally highly excited azulene in supercritical xenon/ethane mixtures

The collisional deactivation of vibrationally highly excited azulene was studied in equimolar supercritical mixtures of xenon and ethane al 385 K from gas to liquid phase densities. Azulene with an energy of 18 000 cm(-1) was generated by laser excitation into the S-1- and subsequent internal conversion to the S-0*-ground state. The loss of vibrational energy was monitored by transient absorption at the red edge of the S-3<--S-0 absorption band at 290 nm. Transient signals were converted into energy-lime 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 10(-3) mol/cm(3), the observed collisional deactivation rate constants k(c) of the mixture were equal to the sum of the individual contributions of ethane and xenon collisions as expected from simple gas kinetics. Ar mixture densities above 2x10(-3) mol/cm(3), k(c) is smaller than the deactivation rate constant found in neat ethane at half the density. This behavior can be rationalized by a model employing an effective collision frequency which is related to the finite lifetime of collision complexes; the required parameters follow from experiments in neat xenon and ethane.