Overview
In Mössbauer spectroscopy, γ-radiation is absorbed by a nucleus.1 For an absorption event to be detectable, it needs to occur without recoil, which can be significant due to the high energies of the γ-photons (14.4 keV in the case of57Fe). Unlike in gaseous or liquid samples, the recoil energy in solid materials can be taken up by the environment of the absorbing atom, i.e. it is transferred into vibrational degrees of freedom of the molecule or crystal. The fraction of recoil-free absorption events is given by the Lamb–Mössbauer factor f
\(f=\exp\left[\frac{-\left\langle x^{2}\right\rangle E_{\gamma}^{2}}{\left(\text{ℏc}\right)^{2}}\right]\)(1)
where E γ is the energy of the absorbed γ-photon, <x 2> is the mean square displacement of the nucleus from its equilibrium position, ħ is Planck’s constant divided by 2π, and c is the speed of light. Thef -factor is relevant to the total intensity of the recorded absorption. The measurement of Mössbauer spectra relies on the ingenious application of the Doppler effect to achieve partial to full overlap between the emission and absorption lines of the γ-source and the sample, respectively. Details are given in Ref. 1 and references therein.
The typical Mössbauer doublet sketched in Figure 1B arises as a normal absorption line shape at the position of the diagnostic isomer shift that is split due to a magnetic quadrupole interaction of characteristic magnitude. Both parameters, isomer shift and quadrupole splitting (see Figure 1B), arise from the hyperfine interaction of nuclear magnetic dipole moment and electron charge distribution. A third characteristic, the electric quadrupole interaction, is less relevant in this context and only mentioned for completeness.1