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The vapor phase fluorescence has been studied in the near ultraviolet for a number of benzene mono-derivatives. The fluorescence was excited at vapor pressures of about 10 to 20 mm Hg using in most cases light from condensed sparks. The spark electrodes were selected so that the excitation of the molecules by the light was mainly into the strong absorption region near the O,O band. The fluorescence spectra were photographed on a Bausch and Lomb medium quartz spectrograph using slit widths ranging from 10 to 300μ. The exposure times on Eastman II-O plates were from 6 to 120 hr. The compounds studied are benzene, chlorobenzene, and fluorobenzene. Chlorobenzene was irradiated by light from Cu, Ni, and Bi sparks and the Hg arc, but in no case could any fluorescence be detected. Fluorobenzene, when excited by a tin spark, gives rise to a fluorescence spectrum of about 90 bands in the region 2640–3000A. Prominent ground state fundamental vibrations are found to occur which agree with values from Raman effect measurements. The analysis of the fluorescence spectrum of fluorobenzene is in agreement with that of the absorption spectrum, i.e., the first electronic transition (singlet terms) is an A1↔B1, with the molecules having C2v symmetry. Carbon ring vibrations and hydrogen vibrations appear excited in the fluorescence spectrum. Several progressions of totally symmetric vibrations are identified, and a number of bands are explained as combinations of fundamental vibrations. Symmetry assignments are suggested for some of the fundamental vibrations.
© 1950 Optical Society of America
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