The collisional deactivation of vibrationally highly excited azulene was studied from gas into compressed Liquid phase by pump-and-probe picosecond laser spectroscopy. Collisional deactivation rates were compared with solvatochromic shifts Delta nu of the azulene S-3<--S-0 absorption band under identical conditions. Employing supercritical fluids at pressures between 0.03 and 4000 bars and temperatures between 298 and 640 K, measurements covering the complete gas-liquid transition were performed. For the energy transfer experiments, azulene with an energy of similar to 20000 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 following the transient absorption at the red wing 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 energy decays were found to be exponential with phenomenological 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 starts 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 propane 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 molecule. 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 hard-sphere particle in a Lennard-Jones fluid. Within this model, average energies [Delta E] transferred per ethane-azulene collision are temperature independent between 298 and 640 K and pressure independent between 0.03 and 4000 bars. By means of radial distribution functions the density dependence of Delta nu can be represented as well. (C) 1997 American Institute of Physics. [S0021-9606(97)01344-5].