molecular electron spectroscopy otherwise UV-spectroscopy (rus. спектроскопия, молекулярная электронная otherwise УФ-спектроскопия) — a method to determine geometrical structure of molecules by their microwave spectra associated with transitions between rotational energy levels.

Description

The energy of a molecule's electrons movement, in accordance with the quantum mechanics principles, is equal to certain discrete values. Having absorbed photons, electrons transit into a state with higher energy; the so called excitation occurs. Depending on how high the absorbed photon energy is, electrons can transit from the state with the lowest energy (ground state) to the first, second, third, etc. excited electronic states up to the point where the excited electron energy exceeds the ionisation potential. In this case, the electron detaches from the molecule and ionisation occurs. Here, different electronic states may correspond to different equilibrium nuclear configurations (for details see the "electron-vibrational spectroscopy" article). Similarly, the transition of electrons from excited levels to lower energy levels is accompanied by the emission of photons.

For most molecules, wavelengths corresponding to electronic transitions extend from the visible light to the ultraviolet (UV) ranges, which is where the second name of the method, UV spectroscopy, comes from.

Electron spectroscopy makes it possible to detect the presence of certain structural groups (called chromophores) in molecules with high accuracy. The characteristic electronic spectra of such chromophores are well known by previous studies. The absorption electron spectroscopy method is very sensitive, being capable of obtaining distinct absorption bands even at low concentrations of the studied substance. Because of this, the method is more often used for qualitative analysis of the molecular structure, although it can also be used for quantitative analysis of the extinction coefficient ε (usually, per mole of substance).

Due to the high sensitivity of electron spectroscopy, recording of spectra at a single pass of light through the sample cell has become widespread as one of the main express methods to analyse material samples in the chemical industry. For a more precise structural analysis of matter, electron microscopy data should be supplemented by the results of vibrational spectroscopy.

Illustrations

Electron absorption spectra: 1 — for benzene (in cyclohexane) and 2 — pyridine (in alcohol) [1]
Electron absorption spectra: 1 — for benzene (in cyclohexane) and 2 — pyridine (in alcohol) [1].

Author

  • Lourie Sergey

Source

  1. Pentin Yu. A., Vilkov L. V. Physical Methods in Chemistry. Textbook (in Russian) // Moscow: Mir, 2003. - 683 pp.

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