External Heavy-Atom Spin—Orbital Coupling Effect. IV. Intersystem Crossing

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Abstract
The relative phosphorescence to fluorescence quantum yields and phosphorescence lifetimes of naphthalene have been measured in a number of glassy media, some of which contain perturbing atoms of large atomic number. The intersystem crossing process S′→T is very sensitive to external heavy-atom perturbation. It is shown that the order of decreasing sensitivity to this perturbation is: intersystem crossing process (S′→T)>phosphorescence quenching process (T→S)>phosphorescence emission process (T→S). It is strongly suggested by the data contained herein that heavy atom quenching of fluorescence can indeed be attributed primarily, if not entirely, to increase in the probability of the intersystem crossing process. A number of possible uses for the heavy-atom technique are suggested.