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The phosphorescence spectrum of p-xylene-hl0 in a p-xylene-dl0 host is first presented. Secondly we report E.S.R. experiments performed on p-xylene-~,~'-d6 in p-xylene-dl0 ; from an analysis of the orientations of the principal axes in combination with the crystal structure and of the hyperfine splittings it is concluded that the in-plane principal axes of the zfs tensor make an angle of 68 ~ with the molecular axes. Thirdly we discuss MicrowaveInduced Delayed Phosphorescence (MIDP) experiments on p-xylene-hl0 in p-xylene-dl0 in which the relative radiative rate constants for decay from the zero-field spin components of the triplet state to single vibronic bands of the ground state have been obtained.The results of the MIDP and E.S.R. measurements are shown to be mutually consistent in terms of a model in which the effect of the crystal field and the methyl substituents on the pseudo-Jahn-Teller unstable electronic structure of the triplet state of benzene is described.

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