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A series of substituted anthracenes covering the complete range of photochemical fluorescence and triplet quantum yields, c p ~ and c p ~, have been examined in cyclohexane, benzene and dioxan solutions by pulse radiolysis. Extinction coefficients have been determined for triplet-triplet absorptions by means of Land's method and these have been used to obtain G-values for triplet states. Values for G (fluorescence) and hence G (total singlet) relative to a standard have also been measured. G (triplet) is independent of c p ~ for cyclohexane solutions, but is dependent on C ~T for benzene and dioxan. G (fluorescence) is approximately proportional to (PF, which implies constancy of G (total singlet) for a given solvent. The mechanistic implications of these findings are discussed.Divergent views have been expressed 1-4 as to the mechanism of formation of solute excited singlet and triplet states during pulse radiolysis of solutions of molecules such as naphthalene in hydrocarbon solvents, particularly cyclohexane. Two main points are at issue. The first is the extent of production of the large yield of solute triplets by inter-system crossing (I.S.C.) from solute singlet states. Direct measurements of G (fluorescence) of naphthalene in cyclohexane indicate that at some concentrations (5 x 10-3-10-2 M) over 40 % of observed triplets are formed by I.S.C. The importance of this pathway in this system is illustrated by the further observation of a 27 % enhancement of G (triplet) on saturating a 0.1 M solution with xenon.2 Nanosecond (nsec) pulse experiments with these solutions show that the triplets are formed in two stages, i.e., a '' rapid " stage complete in a few nsec and responsible for 67-90 % of the total yield, depending on solute concentration, and a "slow " stage with t+ -80 nsec. Most of the yield of triplet naphthalene produced in benzene solution is also of the " rapid " variety. " Rapid " triplets cannot be produced by

Yields of solute radicals and triplet and singlet excited states differ markedly for solutions of naphthalene in two ethers, 1,2-dirnethoxyethane (DME) and tetrahydrofuran (THF), which have almost the same dielectric constant (7.2 and 7-4 respectively at 25°C). The concentration dependences reveal that in both solvents the production of radicals and excited states are not inter-related in a simple manner ; production of singlet and triplet states do, however, seem to be related.Considerable attention has been focused on the yield of ionic intermediates in the radiolysis of liquids. The production of the spectrum of eG in pulse radiolysis of water is convincing evidence for an extensive degree of ionization. Lower yields of e&.. are obtained on pulse radiolysis of alcohols and other moderately polar liquids and addition to methanol of potent electron scavenger^,^ e.g., biphenyl

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