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Phosphorescence microwave double resonance experiments are reported on Zn porphin at 1 '2 K. In glassy solution very broad resonance transitions are observed. However, for Zn porphin in a crystalline n-octane matrix--a system known for its sharp optical spectra (Shpolskii effect)--three pairs of microwave transitions with widths of a few MHz are found, all of them corresponding to a decrease in phosphorescence intensity. By studying the behaviour of the signals for various methods of preparation of the sample and as a function of the optical bandwidth of excitation and detection, one pair of transitions could be assigned to monomeric solute molecules. The corresponding zero-field splittingsare IX-Z[ =1355, ] Y-Z[ =806 MHz. Itwas further established that by ' pumping ' either of these transitions a third one can be detected at the difference frequency, so that the order of the levels must be X> Y> Z (or reverse). The results indicate that the molecule no longer possesses a fourfold axis in the excited state. What one observes must be the lowest vibronic level of a Jahn-Teller unstable state, the degeneracy of which is removed by an anisotropic crystal field. From some preliminary E.S.R. experiments on zinc porphin in an EPA glass at 77 K it is concluded that Z corresponds to that component where the angular momentum lies in the molecular plane; for a ~r~r* electronic state this is expected to be the lowest in energy of the three.

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