Auger effect

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Schematic representation of the Auger effect (KLM Auger process)

The Auger effect [ oʒe -], named after Pierre Auger , is a so-called radiationless transition in the electron shell of an excited atom . The prerequisite is that an unoccupied electron state ( hole ) is present in an inner electron shell within an atom . If it is reoccupied by an electron from an outer shell, the released energy can be transferred to another electron of the same atom, so that it leaves the atom as an Auger electron . Lise Meitner had already described this effect four years before Auger , but little attention was paid to her work. Since the two researchers identified the effect independently of one another, the effect is also referred to as the Auger-Meitner effect in some recent publications .

The effect is used, among other things, in Auger electron spectroscopy (AES).

description

To trigger the Auger effect, the atom must first lose one of its more firmly bound electrons. This can be done artificially through interaction with photons or electrons of sufficient energy ( ionization of an inner shell ) or of course through electron capture . If the vacated space is occupied again by an electron from an outer shell of the atom, the energy released is transferred to another electron in the Auger effect, which was only weakly bound in the atom and is now emitted with a certain kinetic energy. The Auger effect is only possible if the atom has electrons with suitable energies. It then competes with a radiation transition in which no electron is emitted, but a photon of the characteristic X-ray radiation is generated. With the lighter elements, the Auger effect clearly predominates if it is not excluded from an energetic point of view.

The energy of the Auger electron is determined from the energy levels of the original atom and the remaining ion. It is approximately assumed that the individual electrons in the atom or ion can each occupy one of the well-defined energy levels whose position is determined by the nuclear charge, i.e. the atom's chemical atomic number . Then one has to consider three such levels: the energy of the original hole state, the energy of the level from which an electron crosses into the hole, and the energy of the level from which the Auger electron that is finally emitted originates. The possible transitions are therefore named after the three involved electron levels. For example, if there is a hole in the K-shell that is filled by an electron from the L-shell, and an electron from the M-shell is emitted (see figure), this transition is known as the KLM-Auger process. For a more precise description, it should also be taken into account that the energy levels shift somewhat when their occupation with electrons changes. The kinetic energy of the electron emitted in this way clearly shows which element the atom belongs to. However, chemical analysis by means of Auger electron spectroscopy is practically limited to lighter elements, because the Auger effect decreases strongly in favor of the radiation transition with increasing atomic number.

Due to the law of conservation of energy , the Auger electron has exactly the same energy as if it had been knocked out by a photo effect, caused by a photon of the characteristic X-ray radiation that was generated in the same atom when the original hole was filled. For the course of the process, however, this is an inadmissible idea, because no real photon is generated with the Auger effect. Rather, the Auger effect works like an elastic collision between two electrons: beforehand they are bound in the atom, then one is in an energetically lower level and the other is in the state of a free particle. Since no real photon has to be generated, the transition rate of the Auger effect is typically several orders of magnitude higher than with the competing radiation transition . In addition, the Auger effect can also violate the selection rules that apply to the generation of a photon . On the other hand, such collisions can only really take place if the possible final states are not already occupied by electrons ( Pauli principle ).

The extremely fast Coster-Kronig transition is a special case of the Auger process , for example L 1 L 2 M. The original hole L 1 is filled from a higher sub- shell L 2 of the same main shell L. If the Auger electron emitted in the process also comes from the same main shell, then one speaks of the Super-Coster-Kronig transition , for example L 1 L 2 L 3 . Extremely short lifetimes down to 10 −16  s are observed here, which can be understood from the high probability of a collision between two electrons in the same shell. The Coster-Kronig process was named after the two physicists Dirk Coster and Ralph Kronig .

Eye neutralization and Coulomb decay

A process analogous to the normal, inner-atomic Auger effect can also take place with the participation of a solid or another atom. The first case occurs with low-energy ion scattering , one of the standard methods for studying solid surfaces. It is known as eye neutralization because the approaching ion captures an electron from the solid, whereby the binding energy that is released is transferred to another electron in the solid without a real photon being generated in the meantime. Whether this electron can then leave the solid depends on the specific circumstances, in particular on the size of the binding energy released by the capture.

The Auger effect with the participation of a second atom is the basis of the interatomic Coulomb decay of molecules. If one of the atoms in the molecule is ionized by tearing a tightly bound electron away from it, one of its less tightly bound electrons can fill the hole, whereby the binding energy released is transferred to an electron of the other atom without the detour via a real photon so that it flies away. As a result, there are then two ionized atoms that move away from each other due to Coulomb repulsion. From the kinetic energies of the two electrons and two ions, detailed conclusions can be drawn about how the molecule was originally built.

See also

Individual evidence

  1. ↑ Pierre Auger: Sur les rayons β secondaires produits in un gaz par des rayons . In: Comptes Rendus . tape 180 , 1925, pp. 65 ( digitized on Gallica ).
  2. ^ Pierre Auger: Sur L'effet Photoélectrique Composé . In: Journal de Physique et Le Radium . No. 6 , 1925, pp. 205–208 (French, archives-ouvertes.fr [PDF]).
  3. ^ Pierre Auger: L'effet photoélectrique. Dissertation, University of Paris, 1926
  4. ↑ Lise Meitner: About the β-ray spectra and its connection with γ-radiation . In: Journal of Physics A Hadrons and Nuclei . No. 11 , 1922, ISSN  0939-7922 , p. 35-54 , doi : 10.1007 / BF01328399 .
  5. ↑ Ludwig Bergmann, Clemens Schaefer, Wilhelm Raith, with contributions from H. Kleinpoppen , M. Fink, N. Risch: Components of matter: Atoms, molecules, atomic nuclei, elementary particles . Walter de Gruyter, 2003, ISBN 978-3-11-016800-6 , p. 135-136 .
  6. ^ HH Brongersma, M. Draxler, M. de Ridder, P. Bauer: Surface composition analysis by low-energy ion scattering . In: Surface Science Reports . tape 62 , no. 3 , 2007, p. 63-109 , doi : 10.1016 / j.surfrep.2006.12.002 .
  7. ↑ Till Jahnke: Knowledge through decay: Interatomic Coulombic Decay - the subtle side of the Coulomb interaction . In: Physics Journal . tape 13 , no. 9 , 2014, p. 55-58 .