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A yellow color is produced when two colorless pure liquids, α-chloronaphthalene and ethyl iodide, are mixed, although no chemical reaction occurs. Spectroscopic examination reveals that the lowest singlet→triplet absorption band of the naphthalene molecule is greatly enhanced in intensity in the presence of the iodide, and that the development of this absorption band is responsible for the color effect. The process is interpreted in terms of a collisional perturbation of spin-orbital coupling in the π-electron orbitals of the naphthalene. The significance of this phenomenon in several topics in spectroscopy and chemistry is discussed. In particular, the molecular mechanism of heavy atom quenching of fluorescence by foreign species is resolved. A novel Beer's law anomaly is predicted for intercombinations.
Michael Kasha (Tue,) studied this question.