We have obtained and analyzed the absorption, fluorescence, and fluorescence excitation spectra of indole vapor, N-acetyl-L-tryptophan vapor, and 3-indole aldehyde vapor. From analysis of the dependence of the fluorescence spectrum of the free indole molecules on the wavelength of the exciting radiation λex, it follows that emission of fluorescence occurs when the molecules undergo a transition from the one electronically ex- 1cited state Lb. The fluorescence spectra of the studied compounds are insignificantly different, suggesting a major role for the indole chromophore in formation of the compounds. The absorption spectrum of N-acetyl- L-tryptophan, in which the group of atoms is added to the indole ring through a –C–C bond, is similar to the spectrum of indole, while the spectrum of 3-indole aldehyde is significantly different from the indole spec- trum due to the effect of the C=O group conjugated with the indole ring. The fluorescence excitation spectra are considerably different from the absorption spectra. This is associated with the strong dependence of the quantum yield for the free molecules on λex. Qualitatively, they are mirror-symmetric to the fluorescence spec- tra of the stodied compounds. Analysis of the data obtained provides a basis for assuming that in the case of 1free molecules of indole and its derivatives, the La absorption in the extreme long-wavelength region of the 1spectrum does not overlap Lb absorption.
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