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The vibrational relaxation of 1,3,5‐cycloheptatriene (CHT) in liquids, compressed gases and supercritical fluids was studied. Highly vibrationally excited CHT molecules were prepared by excitation with a 248 nm laser pulse and subsequent ultrafast internal conversion. The decay of the internal energy was monitored by ‘‘hot UV absorption spectroscopy’’ using probe pulses at 225 nm and 313 nm generated by stimulated Raman scattering. The experiments show that the CHT internal energy decays monoexponentially with time. In gases at pressures up to 30 bars, the rate constant of the relaxation agrees with the values that can be extrapolated from low density energy transfer experiments. At higher densities, the increase of the relaxation rate deviates markedly from scaling behaviour with density or the inverse diffusion coefficient. The energy transfer in liquids is slower by a factor of three to four than expected from standard isolated binary collision (IBC) models. At present, there is no established theoretical approach that can explain these results.

We have performed time resolved measurements of azulene S2‐S0 fluorescence at different excitation energies E and at different bath gas pressures. Using the S2‐S0 internal conversion rates kIC(E) obtained from low pressure experiments, all nonexponential decays could be modeled by a simple master equation analysis. This presents a new experimental method to obtain parameters which quantitatively describe the efficiency of collisional energy transfer for molecules in excited electronic states. We have further used conventional pump probe spectroscopy to study the collisional deactivation of vibrationally highly excited ground state azulene. This study covers the whole density range from gases to compressed liquids. Deviations from standard isolated binary collision models were detected at surprisingly low densities.