Inelastic collisions leading to a degradation of the internal vibronic energy of an electronically excited molecule (β-naphthylamine) have been observed by direct timing of the fluorescence from the excited state as a function of pressure of added gases. The zero-pressure lifetime spectrum as a function of excitation energy was employed as the energy scale calibration for the lengthening of the fluorescence lifetime. These experiments were performed also as a function of excitation energy and the results interpreted in terms of a collisional inefficiency when compared to a gas-kinetic cross section and a statistical model of energy transfer. Principally helium, argon, and propylene were studied, but preliminary experiments with propane revealed results similar to those for propylene. Helium, as expected, was less efficient than argon which, in turn, due to its lack of internal degrees of freedom was much less efficient than propylene. When the statistical complexions resulting from these added degrees of freedom were computed, it was found that the remaining ``one-dimensional'' efficiency of propylene was quite similar to that of argon. In view of their similar mass this would lend support to a previously advanced model in which simple collision mechanics determine energy transfer in one dimension.

von Weyssenhoff, H., & Schlag, E. W. (1973). Vibrational relaxation by collisions of polyatomic molecules in excited states. The Journal of Chemical Physics, 59(2), 729–740.