Auger electron spectroscopy
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Auger electron spectroscopy is an analytical technique in surface science and materials science.
Auger electron spectroscopy probes the composition and chemistry of a surface by measuring the energy of electrons emitted from that surface when it is irradiated with electrons of energy in the range 2–50 keV. Some of the electrons emitted from the surface have energies characteristic of the element from which they were emitted, and in some cases, the bonding state of those atoms. The physical process which causes these electrons to be emitted is called the Auger effect.
This effect was discovered independently by both Lise Meitner and Pierre Auger in the 1920s. Though the discovery was made and reported by Meitner in 1923 in the journal Zeitschrift für Physik two years before Auger discovered the effect, the English speaking scientific community came to attach Auger's name to it.
The Auger effect occurs because the incident electrons can remove a core state electron from a surface atom. This core state can be filled by an outer shell electron from the same atom, in which case the electron moves to a lower energy state, and the energy associated with the transition is the difference in orbital energies. This energy must be released in some fashion. In some cases this energy is imparted to a second outer shell electron, which then is ejected from the atom. The characteristic energy of this ejected electron is
- ECore State = ES1 + ES2
where S1 and S2 are the outer shell states. Because these orbital energies are determined by the element of the atom, the composition of a surface can be determined.
The emission of a characteristic Auger electron is more probable for lighter elements than heavy elements (where electrons are more tightly bound to the nucleus). Therefore it is more sensitive to the lighter elements, and unlike the competing X-ray fluorescence transition, it can detect elements as light as lithium. In practice the Auger spectrometer can detect elements from lithium (Z = 3) to uranium (Z = 92) and beyond (some transuranic elements, particularly americium (Z = 95), have been analysed). Also Auger spectrometers can operate in resolution mode allowing the investigation of chemical shift — the chemical environment of the elements.
For Auger spectroscopy to be conducted the specimen chamber and spectrometer must be maintained at Ultra High Vacuum (UHV), as any gasses present will both absorb and scatter the very low energy Auger electrons as well as forming a thin 'gas layer' on the surface of the specimen degrading analytical performance.
Auger electron spectroscopy is useful as a surface analytical technique because the energies of the electrons emitted are typically in the range of 50 eV to 3 keV, and at this energy they cannot escape from more than a few nanometers deep in the surface (of course, the higher the energy, the thicker the layer from which they can escape). The technique is commonly used in conjunction with another technique like scanning electron microscopy (SEM) or low energy electron diffraction (LEED).
There are a number of electron microscopes that have been specifically designed to conduct Auger spectroscopy; these are termed scanning Auger microscopes (SAM) and can produce high resolution spatially resolved chemical images. One of greatest problems encountered when conducting SAM investigations is charging of non-conducting specimens, which often restricts the application of the technique to metals. Charging can be controlled by altering the position of the specimen with regards to the incident electron beam, or by the use of an argon ion charge neutralising gun.
Auger is pronounced "O-Zhay", as with French pronunciation.
A technique called sputtering is sometimes used with Auger spectroscopy to remove a thin outer layer of a surface so that Auger electron spectroscopy can be used to study a layer underneath.
[edit] See also
- UPS, Ultra-violet photoelectron spectroscopy (for gases, aka PES)
- PES, Photoemission spectroscopy (for solid surfaces, aka UPS)
- ZEKE, Zero Electron Kinetic Energy spectroscopy
- XPS, X-ray photoelectron spectroscopy
Literature references:
Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, ed. John T. Grant and David Briggs, published by IM Publications, 2003, Chichester, UK
