Nuclear Gamma Resonance (NGR) or Mössbauer spectroscopy

Metastable 57Co nuclei undergo a nuclear electron capture (EC) process that yields 57Fe nuclei in an excited state with an I = 5/2 nuclear spin.

Metastable 57Co nuclei undergo a nuclear electron capture (EC) process that yields 57Fe nuclei in an excited state with an I = 5/2 nuclear spin. Subsequent decay of these excited nuclei to their nuclear ground state occurs either directly (11%) or in a two-step process, from the excited I = 5/2 state to another I = 3/2 excited state and finally from the I=3/2 to the I = 1/2 nuclear ground state. In the last step of the later process, a gamma quantum with a14.41 keV energy is emitted. A sizable fraction (Debye-Waller factor) of these 14.4 keV nuclear emissions occurs without a concomitant lattice phonon excitation. Consequently, the recoil energy becomes negligible and thus leads to an unprecedentedly narrow emission band width of 4.6·10-9 eV, some 13 orders of magnitude less than the gamma quantum energy itself. Mössbauer spectroscopy relies on the resonant absorption of this extraordinarily monochromatic 14.41 keV radiation to probe the nuclear energy levels of 57Fe nuclei (2.2% natural abundance) in a sample of interest. Since the iron atoms of the sample and of the source are generally found in different chemical environments, the resonant absorption of the 14.41keV quanta by 57Fe nuclei in the sample requires a slight change in the radiation frequency emitted by the source. This frequency modulation is achieved by taking advantage of the Doppler effect. The radioactive source is moved versus a stationary sample and the plot of gamma absorption as a function of the source velocity is called a Mössbauer spectrum. Perhaps the most amazing aspect of the phenomenon is the required velocity – it is of the order of just millimeters per second. Some 40 different nuclei exhibit the Mössbauer effect, however an overwhelming majority of these studies are performed on iron-57.

Images & Sample Data

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Important Applications

  • Iron detection, recognition of minerals containing iron
  • Determination of iron valence state, structure and chemical bonding
  • Magnetic properties of compounds and alloys, detection and determination of internal magnetic field in ferromagnets

Many branches of science, including chemistry, biochemistry and geology, take advantage of the Mössbauer spectroscopy.

Three Main Interactions Affecting Mössbauer Spectra

  • Isomer shift – due to the electron density at a nucleus; provides information of the oxidation state, spin state, covalency and electronegativity. It shifts a resonance on the velocity scale without splitting it.
  • Electric quadrupole interaction between the nuclear quadrupole moment and the electric field gradient on the nucleus – depends on oxidation and spin state and on the site symmetry. Splits a resonance line in two.
  • Zeeman interaction between the nuclear spin and magnetic field - monitors magnetic properties like ferromagnetism. Causes six Mössbauer resonances to appear.

Instrumentation

  • Compatible Spectrometers

  • Available Equipment

  • A constant acceleration spectrometer fitted with a flow-type Janis research cryostat and equipped with an 8 T superconducting magnet and a 57Co source, suitable for studying samples containing iron-57. The temperature range is 2 – 220 K.
  • A zero-magnetic field, constant acceleration spectrometer fitted with a heat transfer-type, optical cryostat that allows for measurement of temperature-dependent spectra (2 – 345 K) under constant light irradiation. It is equipped with a 57Co source, suitable for studying samples containing iron-57.

 

Related Publications

Stoian, S.A., et al, Spectroscopic and Theoretical Investigation of a Complex with an O=Fe(IV)-O-Fe(IV)=O Core Related to Methane Monooxygenase Intermediate Q, J. Am. Chem. Soc., 136 (2014) Read online 


Chygorin, E.N., et al, Novel Heterometallic Schiff Base Complexes Featuring Unusual Tetranuclear {CoIII2FeIII2(μ-O)6} and Octanuclear {CoIII4FeIII4(μ-O)14} Cores: Direct Synthesis, Crystal Structures, and Magnetic Properties, Inorg. Chem. 51 (1), 386-396 (2012) Read online 


Tucker, P.C., et al, A Tale of Two Metals: New Cerium Iron Borocarbide Intermetallics Grown from Rare-Earth/Transition Metal Eutectic Fluxes, J. Am. Chem. Soc. 134 (2012) Read online 


Kovnir, K., et al, Spin-Glass Behavior in LaFexCo2-xP2 Solid Solutions: Interplay Between Magnetic Properties and Crystal and Electronic Structures, Inorg. Chem. 50 (2011) Read online 


Białońska, A., et al, A nitrile-rich coordination polymer {[Fe(CH3CN)4(pyrazine)](ClO4)2} exhibits a HS-LS transition, Inorg. Chem. 49 (2010) Read online 


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Last modified on 9 October 2014