Zero-field magnetic resonance study on the phosphorescent triplet state of benzil

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Abstract
The properties of the lowest excited triplet state of benzil in neat crystals have been studied with zero‐field experiments combining microwave spectroscopy and optical detection. Two emitting species, originating from X traps of benzil crystals, were detected. The two X traps were found to have quite different trap depths and zero‐field splittings, yet their triplet kinetics proved to be the same. One of the three spin sublevels almost totally dominates the populating and depopulating activities of the phosphorescent triplet state. A new and more accurate method is described for the determination of the steady‐state population ratios. A discussion of the geometry of the benzil triplet state is given, and it is proposed that benzil assumes a trans‐ (near) planar configuration in the lowest triplet state. Finally, a change in the zero‐field resonance spectrum is reported near a second order phase transition of benzil crystals near 80 °K.

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