electron-vibrational spectroscopy abbr., EVS; VS (rus. спектроскопия, электронно-колебательная otherwise электронно-колебательная спектроскопия молекул abbr., ЭКС) — a type of the high resolution electron spectroscopy method allowing scientists to determine by electronic spectrum vibrational frequencies of ground and excited (usually the lower ones) electronic states which depend on the structure of the substance.

Description

Electron-vibrational spectroscopy is commonly used to study the molecular structure of substances in gas and liquid phases with low vapour pressures and, less often, in solid phase.

To register vibronic spectra, it is equally possible to use both the absorption effect and the phenomenon of fluorescence at resonant or nonresonant excitation of electronic (or more precisely electronic-vibrational) energy levels. In this case, the absorption and fluorescence spectra are complementary to each other.

The absorption spectra are usually obtained by placing a single- or multi-pass cell with the studied substance in the path of a light beam (emitted, for example, by a mercury or halogen lamp) which has a continuous spectrum in the UV/visible range. The light that has passed through the sample is detected with a spectrophotometer.

Fluorescence spectra are recorded with the help of an image amplifier (usually a photomultiplier) that can operate in the broadband mode (fluorescence excitation spectra due to narrowband excitation) or in the photon counting mode.

Due to the fact that the electron-vibrational spectroscopy method studies transitions between different electronic energy levels it makes it possible to record bands not observed in conventional IR or Raman spectra because such bands are forbidden by the selection rules for transitions between energy levels related to vibrational motion of atomic nuclei in a molecule at the same electronic level (ground level) – provided, of course, that such bands are not forbidden by the selection rules for vibronic spectra. The electron-vibrational spectroscopy method is very sensitive and makes it possible to register distinct spectra even at very low concentrations of the substance in the cell. However, it is because of this feature that the electron-vibrational spectroscopy method is poorly applicable, for example, for quantitative analysis of copolymers' composition. The distribution of vibronic bands' intensity depends on the relative positions of potential energy minima in the ground and excited electronic states which enables one, with the use of vibrational and microwave spectroscopy data, to determine the equilibrium configuration of molecules in excited electronic states.

Illustrations

Fragment S1 ← S0 of propanal-d1 electron-vibrational spectrum in the wavelength region of
Fragment S1 ← S0 of propanal-d1 electron-vibrational spectrum in the wavelength region of 341.8 nm with visible "origin" ("0-0" transition) of 29246 cm–1. The spectrum shows two series of electron-vibrational bands, by which, basing on the ab initio calculation data, we may assess such parameters of the structure of molecules in excited electron state as deflection angle of the ethyl fragment relative to scaffold for several stable conformers, as well as the equilibrium angle of C-H bond's departure from the C-C-O plane [3].

Author

  • Lourie Sergey

Sources

  1. Pentin Yu. A., Vilkov L. V. Physical Methods in Chemistry. Textbook (in Russian) // Moscow: Mir, 2003. - 683 pp.
  2. I. S. Godunov et al. Experimental studies of the structure and molecular conformations of carbonyl compounds in basic and lowest excited electronic states (in Russian)// Zhurnal strukturnoj himii. 1995. V. 36, #2. P. 269–286.
  3. I. A. Godunov et al. Analysis of the Vibrational Structure of the Band at 3418 å in the Absorption Electronic Spectrum of Propanal-h1 and -d1 Vapors // Zhurnal fizicheskoi khimii (in Russian). 2005. V. 79, #10. P. 1735–1746.

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