Electron Spectroscopy Analysis

Summary

Electron spectroscopy analysis is a scientific method that uses ionizing radiation, such as ultraviolet radiation, X-ray radiation, and gamma radiation, to eject electrons from atomic and molecular orbitals in a given material. The properties of these electrons are then interpreted to provide information about the system from which they were ejected.

Definition and Basic Principles

The quantum mechanical theory of matter describes the positions and energies of electrons within atoms and molecules. When ionizing radiation is applied to a sample of a material, electrons are ejected from atomic and molecular orbitals in that material. The measured energies of those ejected electrons provide information that corresponds to the chemical identity and molecular structure of the material.

The analytical methods that employ this technique, such as mass spectrometry, typically study the properties of the molecular ions themselves rather than the electrons that were removed. The two processes are related, however, because the energies observed for one technique are often identical to those observed for the other. This can be understood at a rudimentary level by considering the law of conservation of energy as it must apply to the overall process of rearrangement. Electrons move from one orbital to another after one has been removed from an inner orbital and rearrangement of the electron distribution takes place to “fill in the hole.”

Electron spectroscopic methods require that the electron emission process be carried out under high vacuum and with the use of sensitive electronic equipment to capture and measure the emitted electrons and their properties. Each technique utilizes unique methods, but similar devices, to carry out its tasks.

Background and History

The beginning of the science of electron spectroscopy can only be equated to the experiments of British physicist and Nobel laureate J. J. Thomson in 1897. These experiments first identified electrons and protons as the electrically charged particles of which atoms were composed, according to the atomic model propounded by British chemist and physicist Ernest Rutherford. Thomson's experiments were also the first to measure the ratio of the charge of the electron to the mass of the electron. This feature must be known to utilize the interaction of electrons and electromagnetic fields quantitatively.

In 1905, Albert Einstein identified and explained the photoelectric effect, in which light is observed to provide the energy by which electrons are ejected from within atoms. This work, one of only a handful of scientific papers actually published by Einstein, earned him the Nobel Prize in Physics in 1921.

German physicist Wilhelm Röntgen's discovery of X-rays in 1895 and the subsequent development of the means to precisely control their emission provided an important way to probe the nature of matter. X-rays are designated in the electromagnetic spectrum as an intermediate between ultraviolet light and gamma rays. High-vacuum technology and digital electronic technology combine in the construction of devices that permit the precise measurement of minute changes in the properties of electrons in atoms and molecules.

How It Works

Photoelectron Spectroscopy. Two general categories of photoelectron spectroscopy are commonly used. These are ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Both methods function in precisely the same manner, and both utilize the same devices. The difference between them is that UPS uses ultraviolet radiation as the ionizing method, while XPS uses X-rays to affect ionization.

A typical photoelectron spectrometer consists of a high-vacuum chamber containing a sample target; both are connected to an ionizing radiation emitter and a detection system constructed around a magnetic field. In operation, the vacuum chamber is placed under high vacuum. When the system has been evacuated, the sample is introduced, and the emitter irradiates the sample, bringing about the emission of electrons from atomic or molecular orbitals in the material. The emitted electrons are then free to move through the magnetic field, where they impinge upon the detector.

The ability to precisely control the strength of the magnetic field allows an equally precise measurement of the energy of the emitted electrons. This measured energy must correspond to the energy of the electrons within the atomic or molecular orbitals of the target material, according to the mathematics of quantum mechanical theory, and so provides information about the intimate internal structure of the atoms and molecules in the material. The methodology has been developed such that measurements are obtainable using matter in any phase as a solid, liquid, or gas. Each phase requires its own modification of the general technique.

The direct measurement of emitted electron energies through the use of photomultiplying devices is displacing more complex methods based on magnetic field because of the inherent difficulties of providing adequate magnetic shielding to the ever more sensitive components of the devices.

XPS is also known as electron spectroscopy for chemical analysis, or ESCA. The use of this identifier, however, is becoming less common in practice and in the chemical literature.

Auger Electron Spectroscopy (AES). The Auger electron process is a secondary electron emission process that begins with the normal ejection of a core electron by ultraviolet or X-ray radiation. In the Auger process, electrons from higher energy levels shift to lower levels to fill in the gap left by the emission of the core electron. Excess energy that accrues from the difference in orbital energies as the electrons shift then brings about the secondary emission of an electron from a valence shell. The overall process is in accord with both quantum mechanics and the law of conservation of energy, which requires the total energy of a system before a change occurs to be exactly the same as the total energy of the system after a change occurs.

Unlike UPS and XPS, AES generally utilizes an electron beam to affect core electron emission. Detection of emitted electrons is entirely by direct measurement through photomultiplying devices rather than by any magnetic field methods. The methodology of the technique is otherwise similar to that of UPS and XPS. It is amenable to the study of matter in all phases, except for hydrogen and helium, but is generally valuable for use only with solids as a surface analysis technique. This is true because sample materials must be stable under vacuum at pressures of 10-9 Torr. Also, AES is known to be highly sensitive and capable of fast response.

Electron Spin Resonance (ESR). The principles of ESR are based on an entirely different physical property of electrons in their atomic or molecular orbitals. In quantum mechanics, each electron can exist only in particular states with very specific energies within an atom or molecule.

One of the allowed states is designated as “spin.” In this state, the electron can be thought of as an electrical charge that is physically spinning about an axis, thus generating a magnetic field. Only two orientations are allowed for the magnetic fields generated in this way, and according to the Pauli exclusion principle, pairs of electrons must occupy both states in opposition. This requirement means that ESR can be used only with materials that contain single or unpaired electrons, including ions. When placed in an external magnetic field, the magnetic fields of the single electrons align with the external magnetic field.

Subsequent irradiation with an electromagnetic field fluctuating at microwave frequencies acts to invert the magnetic fields of the electrons. Measurement of the frequencies at which inversion takes place provides specific information about the material's structure. The precise locations of inversion signals depend upon the atomic or molecular structure of the material, as these environments affect the nature of the magnetic field surrounding the electron.

Applications and Products

In application, electron spectroscopy is strictly an analytical methodology, and it serves only as a probe of material composition and properties. It does not serve any other purpose, and all applications and products related to electron spectroscopy are the corresponding spectroscopic analyzers and the ancillary products that support their operation.

Spectroscopic analyzers come in a variety of forms and designs, according to the environment in which they will be expected to function, but more with respect to the nature of the use to which they will be put. These range from machines for routine analysis of a limited range of materials and properties at the low end of the scale, to the complex machines used in high-end research that must be capable of extreme sensitivity and finely detailed analysis.

The applications of electron spectroscopic analysis are, in contrast, wide ranging and are applicable in many fields. In its roles in those fields, the methodology has enabled some of the most fundamental technology to be found in modern society.

One application in which electron spectroscopy has proven unequaled in its role is submicroscopic surface analysis. Both XPS and AES are the methods of choice in this application because each can probe to a depth of about 30 microns below the actual surface of a solid material, enabling analysts to see and understand the physical and chemical changes that occur in that region.

The surface of a solid typically represents the point of contact with another solid, and physical interaction between the two normally effects some kind of change to those surfaces because of friction, impact, or electrochemical interaction. A good example of this is the tribological study of moving parts in internal combustion engines. In normal operation, a piston fitted with sealing rings moves with a reciprocating motion within a closely fitted cylinder. The rings physically interact with the wall of the cylinder with intense friction, even though well lubricated, under the influence of the high heat produced through the combustion of fuel. At the same time, the top of the piston is subjected to intense pressures and heat from the explosive combustion of the fuel. At an engine revolution rate of 2,400 revolutions per minute (rpm), each cylinder in a four-cylinder internal combustion engine goes through its reciprocating motion six hundred times each minute, or ten times each second.

In turbine and jet engines, for example, parts are subjected to such stress and friction at a rate of hundreds and even thousands of times per second. Engine and automobile manufacturers and developers must understand what happens to the materials used in the corresponding parts under the conditions of operation. Both XPS and AES are used to probe the material effects at these surfaces to develop better formulations and materials and to understand the weaknesses and failure modes of existing materials.

ESR, on the other hand, is used entirely for the study of the chemical nature of materials in the liquid or gaseous phase. In this role, researchers and analysts use the methodology to study the reactions and mechanisms involving single-electron chemical species. This includes the class of compounds known as radicals, which are essentially molecules containing their full complement of electrons but not of atoms. The methyl radical, for example, is basically a molecule of methane (CH₄) that has lost one hydrogen atom. The remaining CH₃ portion is electrically neutral because it has all of the electrons that would normally be present in a neutral molecule of CH₄ but with one of its electrons free to latch on to the first available molecule that comes along.

Radical reactions are understood to be responsible for many effectsaging in living systems, especially humans; atmospheric reactions, especially in the upper atmosphere and ozone layer; the detrimental effects of singlet oxygen; combustion processes; and many others. In biological systems, specially designed molecules are used to tag other nonparamagnetic molecules so that they can be studied by ESR. Such molecules often include a “nitroso” functional group in their structures to provide a paramagnetic radical site that can be monitored by ESR. The production and testing of these specialty chemicals is another area of application.

Social Context and Future Prospects

Electron spectroscopy analysis is a methodology with an important role behind the scenes. The field is neither well-known nor readily recognized. Nevertheless, it is a critical methodology for advancing the understanding of materials and the nature of matter. As such, electron spectroscopy adds to the general wealth of knowledge in ways that permit the development of new materials and processes and advance the understanding of how existing materials function.

One development of electron spectroscopy, known as scanning Auger microscopy (SAM), has the potential to become an extremely valuable technique because of its ability to generate detailed maps of the surface structure of materials at the atomic and molecular level. By tuning SAM to focus on specific elements, the precise distribution of those elements in the surface being examined can be identified and mapped, providing detailed knowledge of the granularity, crystallinity, and other structural details of the material. This is especially valuable in such widely varied fields as metallurgy, geology, and advanced composite materials.

XPS and AES have been applied in various fields and are becoming essential surface analytical methods. These areas include the aerospace and automotive industries, biomedical technology and pharmaceuticals, semiconductors, and electronics, data storage, lighting and photonics, telecommunications, polymer science, and the rapidly growing solar cell and battery technology fields.

Bibliography

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