Auger Spectroscopy

Jun 01, 2009

Some forms of spectroscopy involve actions other than measuring a property of light. In the case of this form of spectroscopy, the energies of emitted electrons are measured.

David W. Ball
In 1923, Austrian physicist Lise Meitner reported on the emission of electrons from atoms that were bombarded with other electrons (1). In 1925, French physicist Pierre Auger reported the same effect, and correctly attributed it to an excited-state atom that is giving off energy by releasing an upper-quantum state electron (2). Because the energies of these electrons are element-specific, it forms the basis of a type of spectroscopy. Auger spectroscopy was born. (Once again, Meitner is shut out. Many science historians believe that she should have shared Otto Hahn's 1944 Nobel Prize in chemistry for the discovery of nuclear fission. The use of "Auger" to name this form of spectroscopy is thus another example of an instance where Meitner might not be getting the credit she deserves.)

Auger (pronounced "oh-ZHAY") spectroscopy can be considered a form of X-ray photoelectron spectroscopy (XPS) (3), at least in one of its guises. For about 30 years after its formal discovery, it actually was thought of as a nuisance in the performance of XPS. However, since the 1950s, technology has advanced to be able to take advantage of the effect as a separate spectroscopic technique (4).

How It Works

Figure 1
Figure 1 shows a schematic of the steps involved in the Auger effect. In step (a), an incoming energy source, which can be a beam of electrons or X-rays, knocks out a core electron (that is, an electron relatively close to the atomic nucleus). This creates an electronically excited atom, as indicated by the asterisk in the right side of step (a). Step (b) shows that an electron from an outer orbital moves down to fill the hole that the first electron left. This still leaves the atom electronically excited. The atom loses this additional energy by emitting an X-ray photon or ejecting an electron from an even higher shell with whatever excess kinetic energy is necessary, leaving behind a (temporarily) 2+ -charged ion, as shown in step (c).

The kinetic energy of the ejected electron, KE, can be approximated by the energy levels of the original electrons involved:

where E 1, E 2, and E 3 are the original energies of the first core electron, the second electron that moves down, and the third electron that gets ejected, respectively. There is supposed to be a correction for the third energy, because it is actually an energy of the ion, not the atom, but these are usually ignored. Because three electrons are involved, Auger spectroscopy is not used to detect hydrogen or helium but can be utilized for any heavier element. Because the energy levels of the elements typically are well-known, Auger spectroscopy can be used to determine the elemental analysis of a sample, either by analysis of the X-rays emitted or the electrons emitted. Because a free electron will not travel a large distance in a solid, Auger spectroscopy is largely a surface technique, allowing users to probe the chemical composition of a surface.

As the atomic number of the elements being analyzed increases, so do the number of possible transitions, suggesting that Auger spectra get hopelessly complicated for larger elements. However, experience shows that most elements show only a few strong signals in their Auger spectra, dominated by transitions between the 1s, 2s, 2p, and occasionally the n = 3 shells.