We report the results of an extensive study of the rate of radiationless decay in very cold, isolated, naphthalene molecules. The experimental method employed, namely, pulsed narrow band laser excitation of a pulsed seeded supersonic free jet, avoids the complications imposed by the sequence congestion generated by the thermal excitation characteristic of ordinary vapor samples. Our analysis of the fluorescence decay includes a method for transforming room temperature quantum yield data to quantum yields characteristic of the molecules in the supersonic free jet. It is concluded that (a) the out of plane vibration 2(b2g) is very likely a promoting mode for intersystem crossing; (b) there is no need to invoke near resonant coupling of levels in the prepared singlet and final triplet manifolds to explain features of the radiationless decay; and (c) in general, the broad features of radiationless decay in naphthalene are very much like those of the corresponding decay in benzene, and can be similarly explained.
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