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Measurements have been made of the concentration dependence of the rise and decay time characteristics of the monomer and excimer fluorescence of deoxygenated solutions of pyrene in cyclohexane at temperatures from 293 to 340 °K. Two independent methods were employed, one using a pulsed light source and a pulse-sampling oscilloscope, and the other a modulated light source and a phase and modulation fluorometer. In conjunction with observations of the monomer and excimer fluorescence quantum efficiencies, the results have been analyzed to determine the six rate parameters which describe the behaviour of the system. Values of 6.8 x 10 -7 and 0.9 x 10 -7 s are obtained for the radiative lifetimes of the monomer and excimer, respectively. Excimer formation is shown to be a diffusion-controlled collision process, in which every collision between excited and unexcited molecules is effective. From the difference in the activation energies for excimer dissociation and formation, the excimer binding energy is found to be 0.34eV.

Measurements have been made of the fluorescence spectra, lifetimes and quantum efficiencies and of the absorption spectra of several organic molecules in dilute solution. These have been used to test the validity of the theoretical relation between the radiative lifetime and the absorption intensity, and of the mirror-image relation between the fluorescence and absorption spectra. The former is found to be valid provided there is no major change in the nuclear configuration between the ground and excited states, but the latter is usually more sensitive to such changes. The effect of a change in nuclear configuration, which is particularly marked in the higher diphenylpolyenes, is to increase the radiative lifetime and to distort the spectral intensity distribution.

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