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68-513: Both XANES and NEXAFS are acceptable terms for the same technique. XANES name was invented in 1980 by Antonio Bianconi to indicate strong absorption peaks in X-ray absorption spectra in condensed matter due to multiple scattering resonances above the ionization energy. The name NEXAFS was introduced in 1983 by Jo Stohr and is synonymous with XANES, but is generally used when applied to surface and molecular science. The fundamental phenomenon underlying XANES

136-466: A K L 1 L 2 , 3 {\displaystyle KL_{1}L_{2,3}} transition, K {\displaystyle K} represents the core level hole, L 1 {\displaystyle L_{1}} the relaxing electron's initial state, and L 2 , 3 {\displaystyle L_{2,3}} the emitted electron's initial energy state. Figure 1(b) illustrates this transition with

204-430: A x {\displaystyle \sigma _{ax}} is calculated for an isolated atom, a simple modification can be made to account for matrix effects: where α is the angle to the surface normal of the incident electron beam; r m can be established empirically and encompasses electron interactions with the matrix such as ionization due to backscattered electrons. Thus the total yield can be written as: Here N x

272-470: A fluorescent photon. The difference between NEXAFS and traditional photoemission experiments is that in photoemission, the initial photoelectron itself is measured, while in NEXAFS the fluorescent photon or Auger electron or an inelastically scattered photoelectron may also be measured. The distinction sounds trivial but is actually significant: in photoemission the final state of the emitted electron captured in

340-432: A carefully tuned bombarding energy (between 1.5 keV and 3 keV). Control of both the angle and energy can subtly alter the number of emitted electrons vis-à-vis the incident electrons and thereby reduce or altogether eliminate sample charging. In addition to charging effects, AES data can be obscured by the presence of characteristic energy losses in a sample and higher order atomic ionization events. Electrons ejected from

408-740: A consequence, the wave function of the system is a complicated object holding a large amount of information , which usually makes exact or analytical calculations impractical or even impossible. This becomes especially clear by a comparison to classical mechanics. Imagine a single particle that can be described with k {\displaystyle k} numbers (take for example a free particle described by its position and velocity vector, resulting in k = 6 {\displaystyle k=6} ). In classical mechanics, n {\displaystyle n} such particles can simply be described by k ⋅ n {\displaystyle k\cdot n} numbers. The dimension of

476-452: A kinetic energy of: where E Core State {\displaystyle E_{\text{Core State}}} , E B {\displaystyle E_{B}} , E C ′ {\displaystyle E_{C}'} are respectively the core level, first outer shell, and second outer shell electron binding energies (measured from the vacuum level) which are taken to be positive. The apostrophe (tic) denotes

544-511: A molecule becomes more protonated, the ionization potentials increase and the kinetic energy of the emitted outer shell electrons decreases. Despite the advantages of high spatial resolution and precise chemical sensitivity attributed to AES, there are several factors that can limit the applicability of this technique, especially when evaluating solid specimens. One of the most common limitations encountered with Auger spectroscopy are charging effects in non-conducting samples. Charging results when

612-496: A natural polarization that can be utilized to great advantage in NEXAFS studies. The commonly studied molecular adsorbates have sigma and pi bonds that may have a particular orientation on a surface. The angle dependence of the x-ray absorption tracks the orientation of resonant bonds due to dipole selection rules . Soft x-ray absorption spectra are usually measured either through the fluorescent yield, in which emitted photons are monitored, or total electron yield, in which

680-503: A sample or exposing it to a dose of reactive gas. In the absorption edge region of metals, the photoelectron is excited to the first unoccupied level above the Fermi level . Therefore, its mean free path in a pure single crystal at zero temperature is as large as infinite, and it remains very large, increasing the energy of the final state up to about 5 eV above the Fermi level. Beyond the role of

748-408: 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

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816-408: A solid will generally undergo multiple scattering events and lose energy in the form of collective electron density oscillations called plasmons . If plasmon losses have energies near that of an Auger peak, the less intense Auger process may become dwarfed by the plasmon peak. As Auger spectra are normally weak and spread over many eV of energy, they are difficult to extract from the background and in

884-508: 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

952-463: Is a form of electron spectroscopy that relies on the Auger effect , based on the analysis of energetic electrons emitted from an excited atom after a series of internal relaxation events. The Auger effect was discovered independently by both Lise Meitner and Pierre Auger in the 1920s. Though the discovery was made by Meitner and initially reported in the journal Zeitschrift für Physik in 1922, Auger

1020-490: Is a general name for a vast category of physical problems pertaining to the properties of microscopic systems made of many interacting particles. Microscopic here implies that quantum mechanics has to be used to provide an accurate description of the system. Many can be anywhere from three to infinity (in the case of a practically infinite, homogeneous or periodic system, such as a crystal ), although three- and four-body systems can be treated by specific means (respectively

1088-681: Is a widely used surface analysis technique that has been successfully applied to many diverse fields ranging from gas phase chemistry to nanostructure characterization. A new class of high-resolving electrostatic energy analyzers, face-field analyzers (FFA) can be used for remote electron spectroscopy of distant surfaces or surfaces with large roughness or even with deep dimples. These instruments are designed as if to be specifically used in combined scanning electron microscopes (SEMs). "FFA" in principle have no perceptible end-fields, which usually distort focusing in most of analysers known, for example, well known CMA. Many-body The many-body problem

1156-501: Is also used extensively as an evaluation tool on and off fab lines in the microelectronics industry, while the versatility and sensitivity of the Auger process makes it a standard analytical tool in research labs. Theoretically, Auger spectra can also be utilized to distinguish between protonation states. When a molecule is protonated or deprotonated, the geometry and electronic structure is changed, and AES spectra reflect this. In general, as

1224-415: 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

1292-404: Is best obtained from comparison of pure samples. There are a number of electron microscopes that have been specifically designed for use in Auger spectroscopy; these are termed scanning Auger microscopes (SAMs) and can produce high resolution, spatially resolved chemical images. SAM images are obtained by stepping a focused electron beam across a sample surface and measuring the intensity of

1360-417: Is characteristic of an environment and valence state hence one of its more common uses is in fingerprinting: if you have a mixture of sites/compounds in a sample you can fit the measured spectra with a linear combinations of NEXAFS spectra of known species and determine the proportion of each site/compound in the sample. One example of such a use is the determination of the oxidation state of the plutonium in

1428-562: Is credited with the discovery in most of the scientific community. Until the early 1950s Auger transitions were considered nuisance effects by spectroscopists, not containing much relevant material information, but studied so as to explain anomalies in X-ray spectroscopy data. Since 1953 however, AES has become a practical and straightforward characterization technique for probing chemical and compositional surface environments and has found applications in metallurgy , gas-phase chemistry, and throughout

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1496-407: Is described by a core hole in the atomic core level and an excited photoelectron. The final state has a very short life time because of the short life-time of the core hole and the short mean free path of the excited photoelectron with kinetic energy in the range around 20-50 eV. The core hole is filled either via an Auger process or by capture of an electron from another shell followed by emission of

1564-430: Is evident in this chart for increasing atomic number. For heavier elements, x-ray yield becomes greater than Auger yield, indicating an increased difficulty in measuring the Auger peaks for large Z-values. Conversely, AES is sensitive to the lighter elements, and unlike X-ray fluorescence , Auger peaks can be detected for elements as light as lithium ( Z = 3). Lithium represents the lower limit for AES sensitivity since

1632-490: Is shown schematically in figure 2. 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. The detection unit and data processing electronics are collectively referred to as

1700-409: Is the absorption of an x-ray photon by condensed matter with the formation of many body excited states characterized by a core hole in a selected atomic core level (refer to the first Figure). In the single-particle theory approximation, the system is separated into one electron in the core levels of the selected atomic species of the system and N-1 passive electrons. In this approximation the final state

1768-459: Is the number of x atoms per volume, λ the electron escape depth, θ the analyzer angle, T the transmission of the analyzer, I(t) the electron excitation flux at depth t , dΩ the solid angle, and δt is the thickness of the layer being probed. Encompassed in these terms, especially the Auger yield, which is related to the transition probability, is the quantum mechanical overlap of the initial and final state wave functions . Precise expressions for

1836-409: Is the total joint density of states of the initial core level with all final states, consistent with conservation rules. The distinction is critical because in spectroscopy final states are more susceptible to many-body effects than initial states, meaning that NEXAFS spectra are more easily calculable than photoemission spectra. Due to the summation over final states, various sum rules are helpful in

1904-461: 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

1972-472: Is usually attributed to multiple ionization events in an atom or ionization cascades in which a series of electrons is emitted as relaxation occurs for core holes of multiple levels. The presence of satellites can distort the true Auger peak and/or small peak shift information due to chemical bonding at the surface. Several studies have been undertaken to further quantify satellite peaks. Despite these sometimes substantial drawbacks, Auger electron spectroscopy

2040-486: The Faddeev and Faddeev–Yakubovsky equations) and are thus sometimes separately classified as few-body systems . In general terms, while the underlying physical laws that govern the motion of each individual particle may (or may not) be simple, the study of the collection of particles can be extremely complex. In such a quantum system, the repeated interactions between particles create quantum correlations, or entanglement. As

2108-506: The fluorescence (x-ray) yield, ω X {\displaystyle \omega _{X}} , by the relation, where W X {\displaystyle W_{X}} is the X-ray transition probability and W A {\displaystyle W_{A}} is the Auger transition probability. Attempts to relate the fluorescence and Auger yields to atomic number have resulted in plots similar to figure 4. A clear transition from electron to photon emission

X-ray absorption near edge structure - Misplaced Pages Continue

2176-407: The microelectronics industry. 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 several  eV to 50 keV, a core state electron can be removed leaving behind a hole. As this

2244-519: The soil at Rocky Flats . The acronym XANES was first used in 1980 during interpretation of multiple scattering resonances spectra measured at the Stanford Synchrotron Radiation Laboratory (SSRL) by A. Bianconi. In 1982 the first paper on the application of XANES for determination of local structural geometrical distortions using multiple scattering theory was published by A. Bianconi, P. J. Durham and J. B. Pendry . In 1983

2312-470: The Auger effect is a "three state" event necessitating at least three electrons. Neither H nor He can be detected with this technique. For K-level based transitions, Auger effects are dominant for Z < 15 while for L- and M-level transitions, AES data can be measured for Z ≤ 50. The yield limits effectively prescribe a cutoff for AES sensitivity, but complex techniques can be utilized to identify heavier elements, such as uranium and americium , using

2380-416: The Auger effect is not the only mechanism available for atomic relaxation, there is a competition between radiative and non-radiative decay processes to be the primary de-excitation pathway. The total transition rate, ω, is a sum of the non-radiative (Auger) and radiative (photon emission) processes. The Auger yield, ω A {\displaystyle \omega _{A}} , is thus related to

2448-463: The Auger effect. Another critical quantity that determines yield of Auger electrons at a detector is the electron impact cross-section. Early approximations (in cm ) of the cross-section were based on the work of Worthington and Tomlin, with b acting as a scaling factor between 0.25 and 0.35, and C a function of the primary electron beam energy, E p {\displaystyle E_{p}} . While this value of σ

2516-479: The Auger energy as: where F ( B C : x ) {\displaystyle F(BC:x)} 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 E i {\displaystyle E_{i}} and compared to peaks in

2584-671: The Auger peak above the background of scattered electrons. The intensity map is correlated to a gray scale on a monitor with whiter areas corresponding to higher element concentration. In addition, sputtering is sometimes used with Auger spectroscopy to perform depth profiling experiments. Sputtering removes thin outer layers of a surface so that AES can be used to determine the underlying composition. Depth profiles are shown as either Auger peak height vs. sputter time or atomic concentration vs. depth. Precise depth milling through sputtering has made profiling an invaluable technique for chemical analysis of nanostructured materials and thin films. AES

2652-449: The Auger peaks. The peak in derivative mode is not the true Auger peak, but rather the point of maximum slope of N(E) , but this concern is usually ignored. 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. Since

2720-463: 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

2788-551: The EXAFS region corresponds to a single scattering regime; while for lower E, λ {\displaystyle \lambda } is larger than interatomic distances and the XANES region is associated with a multiple scattering regime. The absorption peaks of NEXAFS spectra are determined by multiple scattering resonances of the photoelectron excited at the atomic absorption site and scattered by neighbor atoms. The local character of

X-ray absorption near edge structure - Misplaced Pages Continue

2856-401: The NEXAFS region, starting about 5 eV beyond the absorption threshold, because of the low kinetic energy range (5-150 eV) the photoelectron backscattering amplitude by neighbor atoms is very large so that multiple scattering events become dominant in the NEXAFS spectra. The different energy range between NEXAFS and EXAFS can be also explained in a very simple manner by the comparison between

2924-413: The NEXAFS region: formal valence (very difficult to experimentally determine in a nondestructive way); coordination environment (e.g., octahedral, tetrahedral coordination) and subtle geometrical distortions of it. Transitions to bound vacant states just above the Fermi level can be seen. Thus NEXAFS spectra can be used as a probe of the unoccupied band structure of a material. The near-edge structure

2992-438: The binding energies of the i {\displaystyle i} th level in element of atomic number Z and E i ( Z + 1 ) {\displaystyle E_{i}(Z+1)} 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

3060-456: The chemical state of elements which are present in bulk in minute quantities, it has found widespread use in environmental chemistry and geochemistry . The ability of NEXAFS to study buried atoms is due to its integration over all final states including inelastically scattered electrons, as opposed to photoemission and Auger spectroscopy, which study atoms only with a layer or two of the surface. Much chemical information can be extracted from

3128-553: The classical many-body system scales linearly with the number of particles n {\displaystyle n} . In quantum mechanics, however, the many-body-system is in general in a superposition of combinations of single particle states - all the k n {\displaystyle k^{n}} different combinations have to be accounted for. The dimension of the quantum many body system therefore scales exponentially with n {\displaystyle n} , much faster than in classical mechanics. Because

3196-562: The collection current becomes I ( V + k sin ⁡ ( ω t ) ) {\displaystyle I(V+k\sin(\omega t))} . Taylor expanding gives: Using the setup in figure 2, detecting the signal at frequency ω will give a value for I ′ {\displaystyle I'} or d N d E {\displaystyle {\frac {dN}{dE}}} . Plotting in derivative mode also emphasizes Auger fine structure, which appear as small secondary peaks surrounding

3264-473: 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

3332-400: The detector must be an extended, free-electron state. By contrast, in NEXAFS the final state of the photoelectron may be a bound state such as an exciton since the photoelectron itself need not be detected. The effect of measuring fluorescent photons, Auger electrons, and directly emitted electrons is to sum over all possible final states of the photoelectrons, meaning that what NEXAFS measures

3400-434: The electron energy analyzer. Since the intensity of the Auger peaks may be small compared to the noise level of the background, AES is often run in a derivative mode that serves to highlight the peaks by modulating the electron collection current via a small applied AC voltage. Since this Δ V = k sin ⁡ ( ω t ) {\displaystyle \Delta V=k\sin(\omega t)} ,

3468-416: The final states is determined by the short photoelectron mean free path , that is strongly reduced (down to about 0.3 nm at 50 eV) in this energy range because of inelastic scattering of the photoelectron by electron-hole excitations ( excitons ) and collective electronic oscillations of the valence electrons called plasmons . The great power of NEXAFS derives from its elemental specificity. Because

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3536-478: The first NEXAFS paper examining molecules adsorbed on surfaces appeared. The first XAFS paper, describing the intermediate region between EXAFS and XANES, appeared in 1987. Auger electron spectroscopy Auger electron spectroscopy ( AES ; pronounced [oʒe] in French) is a common analytical technique used specifically in the study of surfaces and, more generally, in the area of materials science . It

3604-423: The interpretation of NEXAFS spectra. When the x-ray photon energy resonantly connects a core level with a narrow final state in a solid, such as an exciton, readily identifiable characteristic peaks will appear in the spectrum. These narrow characteristic spectral peaks give the NEXAFS technique a lot of its analytical power as illustrated by the B 1s π* exciton shown in the second Figure. Synchrotron radiation has

3672-443: The issue of charging, though none of them is ideal and still make quantification of AES data difficult. One such technique involves depositing conductive pads near the analysis area to minimize regional charging. However, this type of approach limits SAM applications as well as the amount of sample material available for probing. A related technique involves thinning or "dimpling" a non-conductive layer with Ar ions and then mounting

3740-499: 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: E i ( Z ) {\displaystyle E_{i}(Z)} are

3808-650: The number of secondary electrons leaving the sample is different from the number of incident electrons, giving rise to a net positive or negative electric charge at the surface. Both positive and negative surface charges severely alter the yield of electrons emitted from the sample and hence distort the measured Auger peaks. To complicate matters, neutralization methods employed in other surface analysis techniques, such as secondary ion mass spectrometry (SIMS), are not applicable to AES, as these methods usually involve surface bombardment with either electrons or ions (i.e. flood gun ). Several processes have been developed to combat

3876-413: The photoelectron wavelength λ {\displaystyle \lambda } and the interatomic distance of the photoabsorber-backscatterer pair. The photoelectron kinetic energy is connected with the wavelength λ {\displaystyle \lambda } by the following relation: which means that for high energy the wavelength is shorter than interatomic distances and hence

3944-409: The presence of plasmon losses; deconvolution of the two peaks becomes extremely difficult. For such spectra, additional analysis through chemical sensitive surface techniques like x-ray photoelectron spectroscopy (XPS) is often required to disentangle the peaks. Sometimes an Auger spectrum can also exhibit "satellite" peaks at well-defined off-set energies from the parent peak. Origin of the satellites

4012-408: The primary Auger peak. These secondary peaks, not to be confused with high energy satellites, which are discussed later, arise from the presence of the same element in multiple different chemical states on a surface (i.e. Adsorbate layers) or from relaxation transitions involving valence band electrons of the substrate. Figure 3 illustrates a derivative spectrum from a copper nitride film clearly showing

4080-439: The required numerical expense grows so quickly, simulating the dynamics of more than three quantum-mechanical particles is already infeasible for many physical systems. Thus, many-body theoretical physics most often relies on a set of approximations specific to the problem at hand, and ranks among the most computationally intensive fields of science. In many cases, emergent phenomena may arise which bear little resemblance to

4148-563: 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 that are close together, giving rise to characteristic peaks in an Auger spectrum. KLL and LMM peaks are some of

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4216-499: The sample is connected to ground through an ammeter and the neutralization current is monitored. Because NEXAFS measurements require an intense tunable source of soft x-rays, they are performed at synchrotrons . Because soft x-rays are absorbed by air, the synchrotron radiation travels from the ring in an evacuated beam-line to the end-station where the specimen to be studied is mounted. Specialized beam-lines intended for NEXAFS studies often have additional capabilities such as heating

4284-424: The sample to a conductive backing prior to AES. This method has been debated, with claims that the thinning process leaves elemental artifacts on a surface and/or creates damaged layers that distort bonding and promote chemical mixing in the sample. As a result, the compositional AES data is considered suspect. The most common setup to minimize charging effects includes use of a glancing angle (~10°) electron beam and

4352-461: 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. 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

4420-620: The transition probability, based on first-order perturbation Hamiltonians , can be found in Thompson and Baker. Often, all of these terms are not known, so most analyses compare measured yields with external standards of known composition. Ratios of the acquired data to standards can eliminate common terms, especially experimental setup characteristics and material parameters, and can be used to determine element composition. Comparison techniques work best for samples of homogeneous binary materials or uniform surface layers, while elemental identification

4488-423: The unoccupied density of states and matrix elements in single electron excitations, many-body effects appear as an "infrared singularity" at the absorption threshold in metals. In the absorption edge region of insulators the photoelectron is excited to the first unoccupied level above the chemical potential but the unscreened core hole forms a localized bound state called core exciton . The fine structure in

4556-425: The various elements have different core level energies, NEXAFS permits extraction of the signal from a surface monolayer or even a single buried layer in the presence of a huge background signal. Buried layers are very important in engineering applications, such as magnetic recording media buried beneath a surface lubricant or dopants below an electrode in an integrated circuit . Because NEXAFS can also determine

4624-446: The x-ray absorption spectra in the high energy range extending from about 150 eV beyond the ionization potential is a powerful tool to determine the atomic pair distribution (i.e. interatomic distances) with a time scale of about 10 s. In fact the final state of the excited photoelectron in the high kinetic energy range (150-2000 eV ) is determined only by single backscattering events due to the low amplitude photoelectron scattering. In

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