THE ROLE OF LOCAL-DENSITY IN THE COLLISIONAL DEACTIVATION OF VIBRATIONALLY HIGHLY EXCITED AZULENE IN SUPERCRITICAL FLUIDS

The collisional deactivation of vibrationally highly excited azulene w as studied from gas into compressed Liquid phase by pump-and-probe pic osecond laser spectroscopy. Collisional deactivation rates were compar ed with solvatochromic shifts Delta nu of the azulene S-3<--S-0 absorp tion band under identical conditions. Employing supercritical fluids a t pressures between 0.03 and 4000 bars and temperatures between 298 an d 640 K, measurements covering the complete gas-liquid transition were performed. For the energy transfer experiments, azulene with an energ y of similar to 20000 cm(-1) was generated by laser excitation into th e S-1- and internal conversion to the S-0-ground state. The subsequen t loss of vibrational energy was monitored by following the transient absorption at the red wing of the S-3<--S-0 absorption band near 290 n m. Transient signals were converted into energy-time profiles using ho t band absorption coefficients from shock wave experiments for calibra tion and accounting for solvent shifts of the spectra. Under all condi tions, the energy decays were found to be exponential with phenomenolo gical deactivation rate constants k(c). k(c) and spectral shifts Delta nu showed quite similar density dependences: the low pressure linear increase of both quantities with density rho at higher densities start s to level off, before it finally becomes stronger again. The parallel behavior of energy transfer rate constants and solvent shifts becomes particularly apparent near to the critical point: measurements in pro pane at 3 K above the critical temperature showed that k(c) and Delta nu are essentially constant over a broad density interval near to the critical density. These observations suggest that both quantities are determined by the same local bath gas density around the azulene molec ule. By Monte Carlo simulations it is shown that k(c)(rho) follows an isolated binary collision (IBC) model, if the collision frequency Z is related to the radial distribution function g(r) of an attractive har d-sphere particle in a Lennard-Jones fluid. Within this model, average energies [Delta E] transferred per ethane-azulene collision are tempe rature independent between 298 and 640 K and pressure independent betw een 0.03 and 4000 bars. By means of radial distribution functions the density dependence of Delta nu can be represented as well. (C) 1997 Am erican Institute of Physics. [S0021-9606(97)01344-5].