Mössbauer Spectroscopy -- from Eric Weisstein's World of Physics

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Mössbauer Spectroscopy

The fraction of events which occur without phonon emission or absorption increases greatly at low temperatures. Mössbauer experiments are therefore often performed at the temperature of liquid helium. In order to observe this virtually monochromatic gamma radiation, a substance with a high cross section is needed to absorb the emitted photons. Fe, Sn, and Ir are virtually the only atoms with a large enough cross sections. The re-absorption will only occur if the energy of the gamma ray in the frame of the absorber is the same as that in the emitter within a few parts to 10¹¹, so the same species must also be used for both emitter and absorber.

, which can be alloyed with Fe or Pd, is typically the source for laboratory studies. It decays to with a half-life of days. After s, decays from the state to the state (emitting a gamma ray with energy of 123 keV), then from the state to the state (emitting a 14.4 keV gamma ray). The sample will have a slightly different resonant absorption energy if it is in a different coordination than the source. This can be understood qualitatively by noting that the wavefunctions of s electrons are nonzero at the position of the nucleus, so they may interact with the nucleus and alter the nuclear energy levels. The more d electrons are present, the more the nucleus is shielded from s electrons. This forces the s cloud to expand, reducing the density at the nucleus. So adding d electrons can alter the absorption energy. This effect is known as isomer shift (or sometimes chemical shift). Isomer shifts for Fe are measured relative to Fe in stainless steel, which is defined to have .

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