Mössbauer spectroscopy
This page is provided for people who stumble across
this WWW site and have no knowledge of Mössbauer spectroscopy.
For more detailed information please refer to Fundamentals
of Mössbauer Spectroscopy and the references.
Recoil-free Resonant Gamma-ray Absorption
All nuclei possess excited states, some of which are
accessible from the ground state by photon absorption. Often the
excited states of the absorber are long-lived and the range of photon
energies which will resonantly excite the absorption is extremely
narrow. If strong absorption is to be observed, a significant
fraction of the energy of the source radiation must be within this
range. Such a source may obviously consist of excited nuclei of the
same isotope as the absorber. The excited nuclei may be decay
products of appropriate parent nuclei. It was once thought that
conservation of momentum requires the recoil of the emitting nucleus
and that the photon would not have the full transition energy and
hence would not resonantly excite the absorber. If the nucleus is
free, the recoil momentum and energy are taken by the nucleus itself.
In a solid the momentum and energy go into lattice vibrations, i.e.,
phonons. The temperature dependence of the absorption cross-section
of Ir191 led Mössbauer (1958) to be
the first to realize that a photon could be emitted with the entire
solid recoiling as one rigid mass. The energy lost to the recoil in
this situation is negligible and the emitted photon may resonantly
excite the absorber.
Mössbauer Spectroscopy
Mössbauer spectroscopy is used primarily to
study the electron structure of materials. This resonant absorption
is observed best in isotopes having long-lived, low-lying excited
nuclear energy states. Among all the elements, the largest
recoil-free resonant cross-section occurs for the isotope Iron 57.
The resonant energies are extremely narrow (about 1 part in 10^12).
This extreme resolution allows the observation of the hyperfine
interactions between the nucleus and the surrounding electrons. This
link between the Mössbauer spectrum and the electron structure
of the sample can be exploited in the study of many types of
materials. Fields in which Mössbauer spectroscopy has been
applied include solid-state physics, surface physics, metallurgy,
chemistry, biochemistry, and geology.
More …
|
