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