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Mössbauer Spectroscopy

Mössbauer spectroscopy, or recoil-free nuclear resonance absorption, was discovered in 1958 by a graduate student, Rudolf Mössbauer, who won a Nobel prize for it only three years later.

If a gamma-ray emitted by a radioactive nucleus, such as Fe57, strikes another nucleus of the same kind, it can be absorbed by that nucleus. The maximum absorption effect will only occur if the two nuclei reach a resonance with each other (imagine two identical tuning forks: if you strike one and place it near the other, the second one will also start ringing - this is resonance). Because the gamma-ray emitted by the first nucleus causes a slight recoil of the nucleus that changes the energy of the gamma-ray, this recoil must be compensated for by moving the source nucleus towards and back from the sample nucleus. Thus the name "recoil-free nuclear resonance" absorption.

Plot of a Mössbauer spectrum

This plot of a Mössbauer spectrum shows the absorption versus the velocity of the source. Disregarding the irregularities in the shape of the two peaks for now, you can see that the maximum absorption (at resonance) occurs only at two velocities. The distance between these two velocities is called the quadrupole splitting, and the offset of their average from zero velocity is called the isomer shift. These two parameters can be used to characterize the iron nuclei in a sample with respect to their valence state (+2 or +3) and the structure of the crystal lattice site in which they lay. The irregular shape of the peaks here is caused by the fact that we are actually looking at four peaks, not two, with two of the peaks being much smaller than the other two, and with different Mössbauer parameters.

For a more complete explanation of how the Mössbauer effect works, please consult the "Fundamentals of Mössbauer Spectroscopy page" provided by WEB Research Co.

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