Auger Electron Spectroscopy (AES) General consideration AES is classified as electron in/electron out electron spectroscopy technique. It depends on a phenomenon called Auger Effect Auger Effect The Auger effect is a physical phenomenon in which the transition of an electron in an atom filling in an inner-shell vacancy causes the emission of another electron. When a core electron is removed, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in a release of energy. Although sometimes this energy is released in the form of an emitted photon, the energy can also be transferred to another electron, which is ejected from the atom. This second ejected electron is called an Auger electron, after one of its discoverers, Pierre Victor Auger.

What is the Auger Electron spectroscopy? Auger electron spectroscopy is a common analytical technique used specifically in the study of surfaces, it depends mainly on the Auger effect. Kinetic energy of the Auger electron: Ekin = ECore State − EB − EC' where ECore State, the core level electron energy EB, the first outer shell electron energy EC' second outer shell electron energy Why can AES be used to identify the chemical composition? 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.

Important Notes The utilized energy to create the core hole (i.e. to remove the core electrons) might be x-ray or electrons beams. Generally, the X-ray is evolved by the same technique used to create the core holes. In other words, electrons beam is used to remove the core electrons, then the outer shell electrons go down to fill the new holes liberating a photon energy (X-ray). hv = EA – EB Where A and B are electron shells However, in the AES this photon energy are used to liberate the auger electrons!!!. Generally if the initial core hole is deeper than 10 keV, X-ray can be obtained, otherwise Auger electrons are evolved.

Electrons transition methodology The electrons move among the close shells. ** Thus for a KL1L2,3 transition, K represents the core level hole, L1 the relaxing electron's initial state, and L2,3 the emitted electron's initial energy state. 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 transitions such as LLM. ** Valence band electrons can also fill core holes or be emitted during KVV-type transitions.

Instrumentation ** Surface sensitivity in AES arises from the fact that emitted electrons usually have energies ranging from 50 eV to 3 keV and at these values, electrons have a short mean free path in a solid. ** The escape depth of electrons is therefore localized to within a few nanometers of the target surface, giving AES an extreme sensitivity to surface species.Because of the low energy of Auger electrons, most AES setups are run under ultra-high vacuum (UHV) conditions. ** Such measures prevent electron scattering off of residual gas atoms as well as the formation of a thin "gas (adsorbate) layer" on the surface of the specimen which degrades analytical performance. ** A typical AES setup is shown schematically in the figure. In this configuration, focused electrons are incident on a sample and emitted electrons are deflected into a cylindrical mirror analyzer (CMA). In the detection unit, Auger electrons are multiplied and the signal sent to data processing electronics. Collected Auger electrons are plotted as a function of energy against the broad secondary electron background spectrum.

Quantitative analysis Qualitative and quantitative analyses Qualitative analysis In AES, each element has special AES peak as fingerprint, so the element can be indentified using its standard AES peak. Quantitative analysis Semi-quantitative compositional and element analysis of a sample using AES is dependent on measuring the yield of Auger electrons during a probing event. Electron yield, in turn, depends on several critical parameters such as electron-impact cross-section and fluorescence yield. Accordingly, some mathematical equations are used to perform the quantitative ananlysis

Uses of AES AES can be used to do the following 1. Estimate the qualitative and quantitative analyses of the surface 2. Detect the elemental composition every accurately especially with the low molecular weight elements (however, as same as XPS, AES can not detect H2 and He). 3. Investigate the surface contamination. 4. One important use for AES is the Scanning Auger Microscope SAM, In which the auger electrons are used to draw the image of the surface according to the difference in binding energy.

Comparison between XPS and AES Radiative energy is X-ray It is very good surface analysis technique as it can be used to detect the surface with a thickness of 10-15 nm. Low speed of analysis Low sample damage High accuracy in the quantitative analysis Can provide precise chemical information AES Radiative energy is electrons beam As the incident energy is electron beam with low energy so it has low mean free path compared with the X-ray which makes AES more sensitive to the surface. High speed of analysis High sample damage Low accuracy in the quantitative analysis Give entrusted chemical information

Ultraviolet Photoelectron Spectroscopy (UPS) Ultraviolet photoelectron spectroscopy (UPS) is a method of probing the occupied DOS of the near surface of a material. In this technique, UV photons of energy hv are created in a continuous discharge source by applying a high voltage to a gas (usually He) to cause breakdown. The photons are then targeted onto the sample under investigation. These photons will liberate electrons with sufficient energy, from the sample and into the vacuum where their kinetic energies may be analysed to gain an idea of their energy origin in the density of states. As the photon may penetrate several nanometres into the surface, some bulk states as well as surface electronic states are probed via this technique. As such, the escape depth of the electron involved is also important.

UPS has not high surface sensitivity technique because of 1. The used photoemission can reach the bulk atoms 2. In case of bulk atoms excitation, the emitted electrons can not be entrapped on the surface because usually the UPS liberate the valence electrons, i.e. no strong holes appear in the surface layer. Accordingly, this technique is invoked in these fields Investigation the surface electronic structure – specifically the surface band structure of clean surfaces and ordered atomic adsorption layers on them. Identification of molecular species on surfaces and the characterization of their decomposition and reactions. Study the adsorbate layer on the surface, especially the organic gases adsorbate layers.

Inverse photoemission spectroscopy (IPES) Inverse photoemission spectroscopy (IPES) is a surface science technique used to study the unoccupied electronic structure of surfaces, thin films and adsorbates. A well-collimated beam of electrons of a well defined energy (< 20 eV) is directed at the sample. These electrons couple to high-lying unoccupied electronic states and decay to low-lying unoccupied states, with a subset of these transitions being radiative. The photons emitted in the decay process are detected and an energy spectrum, photon counts vs. incident electron energy, is generated. Due to the low energy of the incident electrons, their penetration depth is only a few atomic layers, making inverse photoemission a particularly surface sensitive technique. Gun Detector

The obtained results Question Can IPES be used alone for surface characterization ? Can it be considered a trustable analyical technique ?