Electron spectroscopy

(spectroscopy) The study of the energy spectra of photoelectrons or Auger electrons emitted from a substance upon bombardment by electromagnetic radiation, electrons, or ions; used to investigate atomic, molecular, or solid-state structure, and in chemical analysis.

A form of spectroscopy which deals with the emission and recording of the electrons which constitute matter—solids, liquids, or gases. The usual form of spectroscopy concerns the emission or absorption of photons (x-rays, ultraviolet rays, visible or microwave wavelengths, and so on). Electron spectra can be excited by x-rays, which is the basis for electron spectroscopy for chemical analysis (ESCA), or by ultraviolet photons, or by ions (electrons). By means of ESCA, complete sets of photoelectron lines can be excited from the internal (core) levels as well as from the external (valence) region. Also, complete sequences of the Auger electron lines are automatically obtained in this mode.

The electron lines in an ESCA spectrum are extremely sharp and well suited for precision measurements. With a high-resolving ESCA spectrometer which has a magnetic or electrostatic focusing dispersive system, the electron lines have widths which are set by the limit caused by the uncertainty principle (the “inherent” widths of atomic levels). With a suitable choice of radiation, electron spectroscopy reproduces directly the electronic level structure from the innermost shells (core electrons) to the atomic surface (valence or conduction band). Furthermore, all elements from hydrogen to the heaviest ones can be studied even if the element occurs together with several other elements and even if the element represents only a small part of the chemical compound.

When applied to solid materials, ESCA is a typical surface spectroscopy with applications to problems such as chemical surface reactions, for example, corrosion or heterogeneous catalysis. ESCA also reproduces bulk matter properties such as valence electron band structures. Electron spectroscopy can supply a detailed knowledge of the valence orbital structure for all molecules which can be brought into gaseous form with pressures of 10⁻⁵ torr (10⁻³ pascal) or more. Under certain conditions, liquids and solutions of various compositions can be studied by ESCA techniques.

A unique feature of ESCA is that, if the exact position of the electron lines characteristic of the various elements in the molecule is measured, the area of inspection can be moved from one atomic species to another in the molecular structure. If the structure of the molecule is known, the charge distribution can be estimated in a simple way by using, for example, the electronegativity concept and assuming certain resonance structures. More sophisticated quantum-chemical treatments can also be applied. Conversely, if, by means of ESCA, the approximate charge distribution is known, conclusions concerning the structure of the molecule can be drawn. See also Atomic structure and spectra; Electron configuration; Molecular orbital theory; Spectroscopy.

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