Time-dependent effects, relaxation and dynamics

The Mossbauer process and the hyperfine interactions have characteristic times and the spectrum observed in any situation depends on whether the properties of the nuclear environment or the position of the nucleus are changing relative to these times. These time-dependent effects 

The influence which relaxation processes and other time-dependent effects have on the Mossbauer spectra is a function of the relative timescales associated with the effects themselves and the timescales of the nuclear transitions and hyperfine interactions. In order to interpret the Mossbauer spectra in terms of time-dependent effects each type of relaxation phenomenon must be considered in the context of the appropriate timescale. Thus, as with the hyperfine interactions themselves, the time dependence results from the interplay of both nuclear and extra-nuclear factors. 

Since the Mossbauer effect is intimately related to any motion of the emitting or absorbing nucleus on either a microscopic or macroscopic scale, Mossbauer spectroscopy provides a potential means by which information on nuclear dynamics, and hence on the dynamics of a system in which the Mossbauer nucleus acts as a probe, can be obtained. Any motion of the Mossbauer nucleus can influence the Mossbauer spectrum in two ways. Firstly, because this motion may be related to the vibrational properties of the system it can influence the recoil-free fraction and hence the absorption intensity of the spectrum itself. Since the absolute absorption intensity is dependent on a large number of other factors, which may be diflicult to determine accurately, any change in recoil-free fraction is most usefully followed as a function of temperature in order to obtain information on the vibrational properties of the system. The second way in which the effects of any motion of the Mossbauer nucleus in the source or absorber are manifested is in the Mossbauer spectroscopic linewidths, as this motion can be thought of as an additional Doppler motion which may partially smear out the resonant absorption. Since the linewidths are also 

The dynamic properties of the Mossbauer nucleus which can be monitored as a result of their effect on the Mossbauer spectrum can arise from the lattice dynamics of the solid in which the nucleus is situated. It can also result from the motion of a localised part of the system, such as a molecular motion, or from motion of the whole system within its environment. As the effect on the spectrum depends only on the actual motion of the nucleus and not on its origin it is not possible to distinguish directly between the possible sources of any such motion. Since the motion is often related to the internal energy of the system under investigation it is frequently studied as a function of the various parameters which determine the behaviour of the system, and particularly the temperature. 

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