beam-foil spectroscopy


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Beam-foil spectroscopy

A technique used in atomic physics to study the structure and dynamics of atomic ions of any element in any state of ionization. For this purpose, a beam of fast ions is sent through a very thin foil. The ion-foil interaction shakes up the electronic shells of the projectile and, after leaving the foil, the ions shed surplus energy by emitting photons, and sometimes electrons. The energies and intensities of these particles yield spectral information on the projectile. See Particle accelerator

The multitude of collisions inside the foil changes the complement of electrons that travel with the projectile ion; some are ejected and others are captured from the target atoms. The ion beam therefore has a different charge-state composition after passage through the foil. (Higher exit charge states are produced at higher incident beam energies.) The beam-foil interaction efficiently populates atomic levels with a high degree of excitation such as multiply excited and core-excited levels. The richness of the resulting spectra yields a great deal of information on atoms and ions, although it is often difficult to resolve the details of line-rich spectra that reflect the complexity of multiply excited systems.

The ion beam travels in a high vacuum before and after transiting the target foil. This environment minimizes collisional perturbation of the ions. The sudden termination of the ion-foil interaction provides an inherently good time resolution to beam-foil spectroscopy. This property of the source permits lifetime measurements as well as the observation of coherent-excitation phenomena such as quantum beats. Because the ion velocity is constant and measurable, it is sufficient to trace the change in intensity of the fluorescence from the ion beam as a function of distance from the foil in order to determine atomic level lifetimes. See Fluorescence

Beam-foil spectroscopy has developed into many variants which now go under the name of fast-beam spectroscopy. For example, a gas target may be used, a laser, a combination of gas or foil and laser, or a target of free electrons in a heavy-ion storage ring. The ion-foil interaction is capable of producing all ionization stages of all elements from negative ions to U91+. The production of the highest ionization stages, however, requires a beam energy of about 500 MeV/nucleon, which can be reached only at the most energetic accelerators. However, since only the relative motion of electrons and ions is important, the same degree of ionization can be reached by use of 250-keV electrons in an electron-beam ion trap (EBIT). The device offers easier ways to attain high spectroscopic precision because the ions are practically at rest. In beam-foil spectroscopy the ions are rapidly moving, which shifts and broadens the spectral lines. This, in turn, causes problems in wavelength calibration and spectral resolution. However, the inherent time resolution of the foil-excited fast-ion-beam source is unique and remains a great asset in time-resolved spectroscopic measurements. See Atomic structure and spectra, Ion sources, Spectroscopy

beam-foil spectroscopy

[′bēm ‚fȯil spek′träs·kə·pē]
(atomic physics)
A method of studying the structure of atoms and ions in which a beam of ions energized in a particle accelerator passes through a thin carbon foil from which the ions emerge with various numbers of electrons removed and in various excited energy levels; the light or Auger electrons emitted in the deexcitation of these levels are then observed by various spectroscopic techniques. Abbreviated BFS.
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