Auger Electron Spectroscopy - Electron Transitions and The Auger Effect

Electron Transitions and The Auger Effect

The Auger effect is an electronic process at the heart of AES resulting from the inter- and intrastate transitions of electrons in an excited atom. When an atom is probed by an external mechanism, such as a photon or a beam of electrons with energies in the range of 2 keV to 50 keV, a core state electron can be removed leaving behind a hole. As this is an unstable state, the core hole can be filled by an outer shell electron, whereby the electron moving to the lower energy level loses an amount of energy equal to the difference in orbital energies. The transition energy can be coupled to a second outer shell electron which will be emitted from the atom if the transferred energy is greater than the orbital binding energy. An emitted electron will have a kinetic energy of:

where, are respectively the core level, first outer shell, and second outer shell electron energies, measured from the vacuum level. The apostrophe (tic) denotes a slight modification to the binding energy of the outer shell electrons due to the ionized nature of the atom; often however, this energy modification is ignored in order to ease calculations. Since orbital energies are unique to an atom of a specific element, analysis of the ejected electrons can yield information about the chemical composition of a surface. Figure 1 illustrates two schematic views of the Auger process.

The types of state-to-state transitions available to electrons during an Auger event are dependent on several factors, ranging from initial excitation energy to relative interaction rates, yet are often dominated by a few characteristic transitions. Because of the interaction between an electron's spin and orbital angular momentum (spin-orbit coupling) and the concomitant energy level splitting for various shells in an atom, there are a variety of transition pathways for filling a core hole. Energy levels are labeled using a number of different schemes such as the j-j coupling method for heavy elements (Z ≥ 75), the Russell-Saunders L-S method for lighter elements (Z < 20), and a combination of both for intermediate elements. The j-j coupling method, which is historically linked to X-ray notation, is almost always used to denote Auger transitions. Thus for a transition, K represents the core level hole, the relaxing electron's initial state, and the emitted electron's initial energy state. Figure 1(b) illustrates this transition with the corresponding spectroscopic notation. The energy level of the core hole will often determine which transition types will be favored. For single energy levels, i.e. K, transitions can occur from the L levels, giving rise to strong KLL type peaks in an Auger spectrum. Higher level transitions can also occur, but are less probable. For multi-level shells, transitions are available from higher energy orbitals (different n, ℓ quantum numbers) or energy levels within the same shell (same n, different ℓ number). The result are transitions of the type LMM and KLL along with faster Coster–Kronig transitions such as LLM. while Coster–Kronig transitions are faster, they are also less energetic and thus harder to locate on an Auger spectrum. As the atomic number Z increases, so too does the number of potential Auger transitions. Fortunately, the strongest electron-electron interactions are between levels which are close together, giving rise to characteristic peaks in an Auger spectrum. KLL and LMM peaks are some of the most commonly identified transitions during surface analysis. Finally, valence band electrons can also fill core holes or be emitted during KVV-type transitions.

Several models, both phenomenological and analytical, have been developed to describe the energetics of Auger transitions. One of the most tractable descriptions, put forth by Jenkins and Chung, estimates the energy of Auger transition ABC as:

are the binding energies of the th level in element of atomic number Z and are the energies of the same levels in the next element up in the periodic table. While useful in practice, a more rigorous model accounting for effects such as screening and relaxation probabilities between energy levels gives the Auger energy as:

where is the energy of interaction between the B and C level holes in a final atomic state x and the R's represent intra- and extra-atomic transition energies accounting for electronic screening. Auger electron energies can be calculated based on measured values of the various and compared to peaks in the secondary electron spectrum in order to identify chemical species. This technique has been used to compile several reference databases used for analysis in current AES setups.