It is known that at low temperatures relaxation between the spin components of a phosphorescent triplet state of an organic molecule may become very slow as compared to their individual rates of decay. Spectacular changes in the intensity emitted can then be induced by suddenly sweeping a microwave field through one of the zero-field resonances during phosphorescence decay. These microwave induced signals arise because molecules are transferred from a populated non-radiative level to an empty radiative one. In this paper a system of experiments based on this phenomenon is presented which enables one to solve the dynamics of populating and depopulating the individual levels of the phosphorescent state. As a demonstration results of experiments on the aza-naphthalenes quinoline and quinoxaline are given.

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Using microwave induced phosphorescence techniques we determined the rate constants for populating and decay of the lowest triplet state of tetramethylpyrazine (TMP) in a durene crystal. The dynamic behaviour of the spin states Ty, and Tz, corresponding to the in-plane spin axes (principal axes of the dipolar coupling tensor) y' and z' proves to be identical : ky, r = kz, r (radiative decay), ku,=kv (absolute decay rates) and Pu,=Pv (populating rates). This result supports the suggestion of de Groot et al., based on E.S.R. experiments, that for the phosphorescent state of TMP in the durene host the in-plane spin axes y' and z' have undergone an appreciable (~45 ~ rotation with respect to the in-plane molecular axes y and z.

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