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Excimers are dimers with associated excited electronic states, dissociative ground states, and structureless emission spectra. Noble and other monatomic gases form atomic excimers. Aromatic molecules form excimers in fluid solutions, liquids, crystals and polymers, at crystal defects, and intramolecularly. Excimer interaction is attributed to configurational mixing of exciton and charge resonance states. The helium excimer and pyrene crystal dimer potential curves are compared.Studies of aromatic excimers discussed include: (i) solution fluorescence kinetics yielding thermodynamic and rate parameters ;(ii) excimer formation by dimer cation neutralization, molecular ion association, (iii) steric effects on photodimer, sandwich dimer and excimer conformations;(iv) environmental effects on excimer radiative and radiationless transitions ;(v) excimer exciton migration and interaction in crystals; and (vi) evidence for triplet excimers. Related photophysical studies on atomic excimers are considered. Saturated amines, which exhibit vapour and solution excimer fluorescence, provide a link between atomic and aromatic excimers. Aromatic molecules form complexes, exciplexes or mixed excimers with different molecules, and noble gas atoms form complexes or exciplexes with different atoms. and triplet-triplet interaction;

The scintillation response S of organic crystals depends on the nature and energy E of the incident iomzing particle, of residual range r. T h e specific fluorescence dS]dr is not in general proportional to the specific energy loss dE/dr. By considering the quenchng effect of the molecules damaged by the particle on the excitons ' produced by it, it is shown that dS/dr = (A dE/dr)/(l + kB dE/dr). A and kB are constants, which have been evaluated for anthracene from observations of S and E, and the range-energy data. Curves are computed for the relative response S of anthracene to electrons, protons, deuterons and a-particles of E u p to 15 M e V , and these are shown to agree closely with the available

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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.