The use of a combination of mass spectrometers and an accelerator to measure the natural abundances of very rare radioactive isotopes. These abundances are frequently lower than parts per trillion. The most important applications of accelerator mass spectrometry are in archeological, geophysical, environmental, and biological studies, such as in radiocarbon dating by the counting of the rare carbon-14 (radiocarbon; ¹⁴C) isotope. See also: Mass spectroscope; Particle accelerator

The use of spectroscopy (the analysis of light as a function of wavelength) as a tool for obtaining observational data on the chemical compositions, physical conditions, and radial velocities of astronomical objects. Astronomical applications of optical spectroscopy from ground-based observatories cover the electromagnetic spectrum from the near-ultraviolet [wavelengths around 0.3 micrometers (μm)] through the visible (0.4–0.7 μm) and into the near-infrared (2 μm). Space-based observatories, such as the Hubble Space Telescope and the Herschel Space Observatory, extend spectroscopic observations from the far-ultraviolet (0.1 μm) to the far-infrared (200 μm). Work at shorter wavelengths (x-ray and gamma-ray spectroscopy) and longer wavelengths (submillimeter and radio wavelengths) requires techniques other than those discussed here. See also: Gamma-ray astronomy; Hubble Space Telescope; Radio astronomy; Submillimeter astronomy; Ultraviolet astronomy; X-ray astronomy

An analytical technique for determining the composition of a sample in terms of the chemical elements present and their quantities or concentrations. Unlike other methods of elemental analysis, however, the sample is decomposed into its constituent atoms which are then probed spectroscopically.

The arrangement of the constituents of an atom and the manner in which they interact to form a system (the atomic structure), and the patterns of light frequencies emitted and absorbed by atoms, whereby this atomic structure may be elucidated (the atomic spectra). Atoms are composed of particles known as protons and neutrons clustered together in a positively charged nucleus, surrounded by negatively charged particles known as electrons (Fig. 1). This structure, as well as its attendant physical properties, has been borne out through theoretical and experimental advances going back more than two centuries. In particular, the measurement of spectra from atoms has enabled the characterization of these fundamental units of matter. See also: Atom; Atomic nucleus; Atomic physics; Atomic theory; Chemistry; Electron; Matter (physics); Neutron; Physics; Proton; Spectrum

A spectrum consisting of groups or bands of closely spaced lines. Band spectra are characteristic of molecular gases or chemical compounds. When the light emitted or absorbed by molecules is viewed through a spectroscope with small dispersion, the spectrum appears to consist of very wide asymmetrical lines called bands. These bands usually have a maximum intensity near one edge, called a band head, and a gradually decreasing intensity on the other side. In some band systems the intensity shading is toward shorter waves, in others toward longer waves. Each band system consists of a series of nearly equally spaced bands called progressions; corresponding bands of different progressions form groups called sequences.

An optical device that exhibits a spatial periodic variation in one or more of the following properties: transmittance, surface profile, or refractive index. Typically, though not necessarily, the variation is one dimensional (only in one direction in the plane of the grating), and the period varies from about one-half to about one hundred times the wavelength of interest. When the variation is two dimensional, the component is normally called a diffractive optical element (DOE).

The study of the resonant response to microwave- or radio-frequency radiation of paramagnetic materials placed in a magnetic field. It is sometimes referred to as electron spin resonance (ESR). Paramagnetic substances normally have an odd number of electrons or unpaired electrons, but sometimes electron paramagnetic resonance (EPR) is observed for ions or biradicals with an even number of electrons. EPR spectra are normally presented as plots of the first derivative of the energy absorbed from an oscillating magnetic field at a fixed microwave frequency versus the magnetic field strength. The dispersion may also be detected.

A form of spectroscopy which deals with the emission and recording of the electrons which constitute matter—solids, liquids, or gases. The usual form of spectroscopy concerns the emission or absorption of photons [x-rays, ultraviolet (uv) rays, visible or microwave wavelengths, and so on]. Electron spectra can be excited by x-rays, which is the basis for electron spectroscopy for chemical analysis (ESCA), or by uv photons, or by ions (electrons; Fig. 1). By means of x-ray or uv photons with energy E_hv, photoelectron spectra (PES) are produced when electrons with binding energies E_b are emitted with energy E_kinetic from bound molecular states, according to the equation below.

The structured absorption on the high-energy side of an x-ray absorption edge. The absorption edges for an element are abrupt increases in x-ray absorption that occur when the energy of the incident x-ray matches the binding energy of a core electron (typically a 1s or a 2p electron). For x-ray energies above the edge energy, a core electron is ejected from the atom. The ejected core electron can be thought of as a spherical wave propagating outward from the absorbing atom. The photoelectron wavelength is determined by its kinetic energy, which is in turn determined by the difference between the incident x-ray energy and the core-electron binding energy. As the x-ray energy increases, the kinetic energy of the photoelectron increases, and thus its wavelength decreases. See also: Absorption of electromagnetic radiation; Light; Photoemission; Quantum mechanics

A term referring to the closely spaced groups of lines observed in the spectra of the lightest elements, notably hydrogen and helium. The components of any one such group are characterized by identical values of the principal quantum number n, but different values of the azimuthal quantum number l and the angular momentum quantum num­ber j.