Yield of singlet and triplet excitons from x-ray and ruby laser excitation of anthracene single crystals

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Abstract
A new method for determining the singlet:triplet yields for x‐ray and ruby laser excited anthracene crystals is described. It was found that the ratio is higher than has been reported for x‐ray excitation and lower than reported for ruby laser excitation. X‐ray excitation produces roughly equal numbers of triplets and singlets. Ruby laser excitation indicates that the intersystem crossing rate from the first excited singlet is 1.5±0.5×107 sec−1, which is larger than had been reported, but still smaller than for anthracene molecules in solution. The greater triplet yield produced by x irradiation is related to efficient spin–lattice relaxation in highly excited electronic states and neither spin–spin nor spin–lattice relaxation in the electron and hole ground states is important in the short time involved in geminate recombination.