We report that the fluorescence lifetime is 38 ± 2 ns and that the oxygen quenching rate constant is 2.49 × 10<sup>9</sup> L mol<sup>−1.</sup>s<sup>−1</sup> for gas-phase fluoranthene excited at 337.1 nm. A fluorescence lifetime of 24 ± 3 ns for excitation at 351 nm has been reported previously by Ehrlich and Wilson [J. Chem. Phys. 67, 5391 (1977)]. According to the current spectroscopic assignments, the 337.1-nm and the 351-nm lasers excite fluoranthene molecules to the same S<sub>2</sub> electronic state with excess vibrational energies of 1900 cm<sup>−1</sup> and 710 cm<sup>−1</sup> respectively. Therefore, our measured lifetime reveals an anomalous trend of the fluorescence lifetime if both the current spectroscopic assignment and the value of 24 ns for the lifetime excited at 351 nm are accepted. Our measured oxygen quenching rate constant indicates that the oxygen quenching rate for fluoranthene is about 100 times slower than that of anthracene and pyrene. The unusually slow oxygen quenching rate in the gas phase may be related to the previously reported unusual inertness of excited fluoranthene molecules in other environments. Possible explanations of these abnormal behaviors of fluoranthene are discussed.

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