Fluorescence of Naphthacene Vapor

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The absorption spectrum of naphthacene vapor is similar to that in solution. There is an absorption band with pronounced vibrational structure running from about 5000 to 3500 Å and a second stronger band from 2700 Å to shorter wavelengths. We have examined the fluorescence spectrum of the vapor at low pressures, such that molecules have no collisions during the lifetime of the excited state. By selective excitation with either the 3650 Å line or the 2537 Å line of the mercury lamp it is possible to excite molecules into either the first excited singlet state or into the second excited singlet state. The fluorescence emission appears to be from the first excited singlet state in both cases even though the molecules suffer no intermolecular collisions in the excited state.