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The lowest energy T1←S0 transitions of the molecules chlorobenzene, 1-chloronaphthalene, anthracene, 9,10-dichloroanthracene, 9,10-dibromoanthracene, naphthacene, pyridine, and phenazine have been investigated in a variety of solvents, some of which contain atoms of large atomic number (henceforth called ``heavy atoms''). It is shown that in heavy-atom solvents an increase of the T1←S0 absorptivity occurs, and the technique of external heavy-atom perturbation is demonstrated as a method of conveniently locating and validating T1←S0 transitions. Monitoring experiments in which nonspin-intercombination transitions, such as S1←S0 transitions of the molecule being investigated, and S1←S0 transitions of low-level contaminants have been subjected to external heavy-atom perturbation, have permitted the conclusion that the method is unique for T1←S0 transitions. It is further verified that the external heavy-atom effect is a spin-orbital perturbation in nature. Much new data on triplet-state energies and lifetimes has been obtained and is contained herein.
McGlynn et al. (Wed,) studied this question.