Abstract

A procedure is suggested for direct recording, in the infrared, of the vibrational spectra of excited electronic states of molecules. The procedure resembles the chemical modulation technique of Bair, Lund, and Cross as well as the pressure modulation technique of Gilfert and Williams. Infrared radiation is passed through a sample but is not chopped. Instead, ultraviolet radiation which also impinges upon the sample is chopped at the tuned frequency of the detector–amplifier system. Only those infrared transitions which take place within the vibrational manifold of the state populated due to the ultraviolet light are recorded. Using a simple adaptation of a commercial spectrometer, this makes possible the recording of the infrared spectra of the lowest triplet state of certain molecules. Preliminary results on naphthalene and acetone are reported and necessary improvements in the instrumentation are discussed. The limitations of the entire procedure are analyzed in terms of a simple model in which the excited molecule is supposed to have only one vibrational degree of freedom and the state manifold for that vibration has only two levels.

© 1963 Optical Society of America

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