External Heavy Atom Perturbation of Vibronic Transitions in Singlet–Triplet Spectra

The high-resolution phosphorescence spectra of a number of compounds were obtained at 4°K both in the presence and absence of external heavy atom perturbers. A comparison of the two cases demonstrates that only totally symmetric modes of the molecule are enhanced by the external heavy atom effect. This result implies that the enhancement mechanism is purely electronic in nature and involves no vibronic coupling. The results also suggest that the external heavy atom effect can be useful in aiding the assignment of complicated singlet–triplet spectra.