The behavior of triplet excitations of a single isotopic guest in a benzene crystal (C6H6in C6D6) is investigated as a function of temperature from 2deg; to 15deg;K by detailed measurements of the phosphorescence. It is found that even in dilute systems there is significant (sim;20percent; for 0.2percent; guest) temperaturehyphen;independent annihilation below 8.0deg;K. Above 8.0deg;K a thermally activated annihilation process dominates the phosphorescence quenching in an ultrapure sample. Thus while the firsthyphen;order decay rate, which includes impurity quenching, increases by less than a factor of 2, the secondhyphen;order annihilation rate increases by five orders of magnitude when the temperature is raised from 4deg; to 12deg;K with an activation energy equal to the known trap depth. The difficulty of fitting a nonexponential decay to the combination of firsthyphen; and secondhyphen;order decays is discussed and a new method is suggested which accomplishes this. Measurements of the temperature dependence of the phosphorescence decay, the steadyhyphen;state phosphorescence intensity, and the steadyhyphen;state delayed fluorescence intensity provide independent evaluations of the annihilation rate which are in good agreement with one another at all temperatures. The low delayedhyphen;fluorescence quantum yield in benzene is discussed.
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