Level anticrossing and cross-relaxation in phosphorescent crystals

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Abstract
Further experimental results on level anticrossing and cross-relaxation effects in phosphorescent benzophenone and 4,4′-dibromobenzophenone crystals are reported. In addition to level anticrossing lines, and triplet-triplet and triplet-radical cross-relaxation lines, the spectra of 4,4′-dibromobezophenone show strong cross-relaxation with the bromine nuclear quadrupole states. A discussion is given of level anticrossing and cross-relaxation in order to explain the features of the spectra. Among others, the peculiar behaviour of the slow-recovery lines, attributed as before to cross-relaxation between the phosphorescent molecules and free hydrogen atoms, is explained by a thermodynamical model of cross-relaxation. While in the experiments mentioned above no r.f. or microwave fields are used, the application of a strong r.f. field gives, for magnetic fields close to the values where cross-relaxation occurs, rise to an effect analogous to the solid effect in N.M.R. These r.f. experiments shed light on the distinction between level anticrossing and cross-relaxation and on the magnetic interactions between the spins in our phosphorescent crystals. Finally it is shown how the present types of effect might be used for obtaining very high degrees of optical nuclear spin polarization.

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