Density dependence of the collisional deactivation of highly vibrationally excited cycloheptatriene in compressed gases, supercritical fluids, and liquids

The collisional deactivation of highly vibrationally excited 1,3,5-cycloheptatriene (CHT) in compressed gases, supercritical fluids, and in liquids was studied. Different bath gases and solvents (alkanes, alcohols, CO2, CHF3, nitrogen) and pressures up to 200 bars were used. This way, measurements covered the complete gas-liquid transition regime. CHT with a vibrational energy of 41 000 cm(-1) was generated by laser excitation into the S-1 state and internal conversion to the S-0 ground state. From picosecond UV absorption measurements, energy-loss profiles were obtained taking into account the density dependent absorption spectra, which were measured separately. A monoexponential decrease of the internal energy in CHT was observed under all studied conditions. In gases at pressures up to 40 bars, the relaxation rates increase linearly with the density and the energies transferred per collision, [Delta E], agree with those values obtained in earlier low pressure gas phase experiments. At higher densities, the relaxation rates deviate markedly from predictions based on simple scaling assumptions with appropriate collision frequencies. The deactivation in liquids is slower by a factor of 3-5 than expected from such isolated binary collision (IBC) models. These results are rationalized using a model that takes into account the finite lifetime of collision complexes. (C) 1997 American Institute of Physics.