Mößbauer spectroscopy

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Mößbauer spectroscopy is a spectroscopic technique based on the Mössbauer effect. In its most common form, Mössbauer Absorption Spectroscopy, a solid sample is exposed to a beam of gamma radiation, and a detector measures the intensity of the beam that is transmitted through the sample, which will change depending on how many gamma rays are absorbed by the sample. The atoms in the source emitting the gamma rays are the same as the atoms in the sample absorbing them. It is thanks to the Mössbauer effect that a significant fraction of the gamma rays emitted by the atoms in the source do not lose any energy due to recoil and thus have almost the right energy to be absorbed by the target atoms. The gamma-ray energy is varied by accelerating the gamma-ray source through a range of velocities with a linear motor. The relative motion between the source and sample results in an energy shift due to the Doppler effect.

In the resulting spectra, gamma-ray intensity is plotted as a function of the source velocity. At velocities corresponding to the resonant energy levels of the sample, some of the gamma-rays are absorbed, resulting in a drop in the measured intensity and a corresponding dip in the spectrum. The number, positions, and intensities of the dips (also called peaks) provide information about the chemical environment of the absorbing nuclei and can be used to characterize the sample.

In order for Mössbauer absorption of gamma-rays to occur, the gamma-ray must be of the appropriate energy for the nuclear transitions of the atoms being probed, which is almost always achieved by having the same atoms of the same isotope in both the source and the target. Also, the gamma-ray energy should be relatively low, otherwise the system will have a low recoil-free fraction (see Mössbauer effect) resulting in a poor signal-to-noise ratio. Only a handful of elemental isotopes exist for which these criteria are met, so Mössbauer spectroscopy can only be applied to a relatively small group of atoms including: ⁵⁷Fe, ¹²⁹I, ¹¹⁹Sn, and ¹²¹Sb. Of these, ⁵⁷Fe is by far the most common element studied using the technique.