2.3.3. Auger Electrons

The Auger process was discovered by Pierre Auger (pronounced "O-jhay") in 1926, when he observed tracks of constant length in a cloud chamber, but Auger electrons were not used to study surfaces until the late 1950's and 60's. Auger electrons are electrons ejected by radiationless excitation of a target atom by the incident electron beam. When an electron from the L shell drops to fill a vacancy formed by K-shell ionization, the resulting X-ray photon with energy EK - EL may not be emitted from the atom. If this photon strikes a lower energy electron (e.g., an M-shell electron), this outer electron may be ejected as a low-energy Auger electron. Auger electrons are characteristic of the fine structure of the atom and have energies between 280 eV (carbon) and 2.1 keV (sulfur). By discriminating between Auger electrons of various energies, a chemical analysis of the specimen surface can be made.

Auger electron energies are closely related to the corresponding X-ray energy, and most usually are described in X-ray notation. For example, the Si KL1L2,3 transition, experimentally observed at 1620 eV, involves removal of an electron in the K shell allowing an electron from the L1 shell to descend with the emission of energy of 1690 eV. This energy can either be emitted as a Si-Ka X-ray, or it can by transferred to a third electron, in this case in the L2,3 shell, which has a binding energy of about 90 eV, ejecting it from the atom with an energy of around 1600 eV.

The probability of Auger electron production increases as the difference between the energy states of the shells decreases. Light elements are more susceptible to the formation of Auger electrons by multiple ionizations. Thus the proportion of radiation emitted at characteristic wavelengths is lower than for heavier elements. The proportion of Auger emission is greater than 0.5 up to about Z = 30 (zinc). So, typically, one switches which transition is used as we move up the periodic table: KLL transitions for light elements, LMM after that, and then MNN.

The Auger phenomenon is described by the fluorescent yield, w, which for K-radiation is defined as:

[Fluorescent Yield Eqn.]

Fluorescent yields for the light elements are generally less than 0.2 for the K-lines. The X-ray yield increases sharply with increasing Z and Auger electron yield decreases (Figures 2.3.3a. and 2.3.3b.). Thus Auger electrons provide a good basis for analysis of light atoms. One might expect that X-ray intensities would be lower at low Z because of increased Auger electron production, but lower fluorescent yield compensates for easier ionization.

[Fluorescent Yield]

Figure 2.3.3a. Schematic variation of fluorescence yields of the K and L spectral lines with atomic number (Z) of the emitting element. The low yields in the low-Z range for each spectral series is a major factor in limiting the sensitivity of X-ray spectrometry in the analysis of light elements.

[Experimental Fluorescent Yields]

Figure 2.3.3b. Experimentally determined K-fluorescence yield as a function of atomic number (after Heinrich 1981).

Auger electrons are produced from depths of about a wavelength into the sample because their low energies make them easily reabsorbed. This makes Auger electrons particularly good for analysis of surface composition, but such analysis requires ultra-high vacuum to avoid absorptive losses. Figure 2.3.3.3. shows SE and Auger yield as a function of energy; the Auger electrons appear as a slight dimple which is often enhanced for detection by taking the derivative of the curve (dN(E)/dE below).

[Auger Electrons]


Figure 2.3.3.3. Auger electron spectra of silver with an incident beam energy of 1 keV. Derivative and integral spectra are compared (after Goldstein et al. 1981).

The production of Auger electrons also produces satellite peaks (see 2.4.3.7. Satellite Peaks).


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Copyright 1997-2003, James H. Wittke

Last update: 01/18/2006 01:47 PM.