Atomic form factor

Atomic form factor

In physics, the atomic form factor, or atomic scattering factor, is a measure of the scattering intensity of a wave by an isolated atom. The atomic form factor depends on the type of scattering, typically X-ray, electron or neutron. For crystals, atomic form factors are used to calculate the structure factor of a unit cell.

X-ray form factor

X-rays are scattered by the electron cloud of the atom and hence the scattering power of x-rays increases with the atomic number of the atoms in a sample. As a result, x-rays are not very sensitive to light atoms, such as hydrogen and helium, and there is very little contrast between elements adjacent to each other in the periodic table. The x-ray form factor is defined as the Fourier transform of the electron charge density.

Electron form factor

Electron form factors can be defined as the Fourier transform of the potential distribution of the atom. [cite book |last=Cowley |first=John M. |authorlink=John M. Cowley |title=Diffraction Physics |year=1981 |publisher=North-Holland Physics Publishing |isbn=0-444-86121-1 |pages=p. 78 ] The electron form factors are normally calculated from X-ray form factors using the Mott-Bethe formula. [cite book |last=De Graef |first=Marc |title=Introduction to Conventional Transmission Electron Microscopy |year=2003 |publisher=Cambridge University Press |isbn=0-521-62995-0 |pages=p. 113 ] This formula takes into account both elastic electron-cloud scattering and elastic nuclear scattering.

Neutron form factor

Neutrons are scattered by the nucleus of the atom but due to their finite magnetic moment they will also interact with the electron clouds of magnetic ions. Neutron form factors are usually described by the neutron scattering length, "b". The neutron scattering length may only be determined experimentally since the theory of nuclear forces is not adequate to calculate or predict "b" from other properties of the nucleus. [Squires, "Introduction to the theory of thermal neutron scattering", Dover Publications (1996) ISBN 048669447X] Neutron scattering lengths vary erractically between neighbouring elements in the periodic table and even between isotopes of the same element. Hence isotopic substitution in neutron diffraction may be used to distinguish between individual atomic sites in a sample. Since the measured intensity of the diffraction patterns are related to the magnitude of the neutron scattering lengths, differences between subsequent diffraction patterns of compositionally identical samples containing different isoptopes may be taken to yield the individual atomic contributions.

References


Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать курсовую

Look at other dictionaries:

  • Form factor — may refer to:* Form factor (radiative transfer) or emissivity, the proportion of energy transmitted by that object which can be transferred to another object *Form factor (electronics), an alternating current waveform *Electric form factor, the… …   Wikipedia

  • Magnetic form factor — In electromagnetism, a magnetic form factor is the Fourier transform of an electric current distribution in space. See also For the form factor relevant to magnetic diffraction of free neutrons by unpaired outer electrons of an atom see also:… …   Wikipedia

  • Structure factor — In physics, in the area of crystallography, the structure factor of a crystal is a mathematical description of how the crystal scatters incident radiation. The structure factor is a particularly useful tool in the interpretation of interference… …   Wikipedia

  • Atomic units — (au) form a system of units convenient for atomic physics, electromagnetism, and quantum electrodynamics, especially when the focus is on the properties of electrons. There are two different kinds of atomic units, which one might name Hartree… …   Wikipedia

  • Atomic orbital — The shapes of the first five atomic orbitals: 1s, 2s, 2px, 2py, and 2pz. The colors show the wave function phase. These are graphs of ψ(x,y,z) functions which depend on the coordinates of one electron. To see the elongated shape of ψ(x,y,z)2… …   Wikipedia

  • Atomic nucleus — A figurative depiction of the helium 4 atom with the electron cloud in shades of gray. In the nucleus, the two protons and two neutrons are depicted in red and blue. This depiction shows the particles as separate, whereas in an actual helium atom …   Wikipedia

  • Atomic clock — Nuclear clock redirects here. For the clock as a measure for risk of catastrophic destruction, see Doomsday Clock. For a clock updated by radio signals (commonly but inaccurately called an atomic clock ), see Radio clock. For the album by Zion I …   Wikipedia

  • Debate over the atomic bombings of Hiroshima and Nagasaki — The Fat Man mushroom cloud resulting from the nuclear explosion over Nagasaki rises 18 km (11 mi, 60,000 ft) into the air from the hypocenter …   Wikipedia

  • List of Atomic Betty characters — This is a list of characters from the animated television series Atomic Betty. Contents 1 Main characters 1.1 Atomic Betty/ Betty Barrett 1.2 Sparky 1.3 Robot X 5 …   Wikipedia

  • List of characters in Atomic Betty — This is a list of characters from the Cartoon Network and Teletoon animated television series Atomic Betty .With the exception of the show s eponymous character and the particular story involved, most of the characters encountered in the series… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”