Introduction

In this chapter the various kinds of time-dependent phenomena that can be studied by Mössbauer spectroscopy will be reviewed and the relationship between the data obtained by Mössbauer spectroscopy and that derived from other techniques will be assessed.

In many discussions on time-dependent or dynamical effects in Mössbauer spectroscopy the essential physical ideas get obscured by complicated theoretical formalism. The ideas, however, are quite simple and are capable of being appreciated at a qualitative level before embarking on detailed calculations. The present chapter is an attempt to fulfil this need and bring out the important concepts at a non-specialist level. Where necessary, the reader may refer to the original sources cited for more elaborate treatments. For these same reasons, only a few simple models have been chosen, which can be analysed without a detailed mathematical treatment.

As briefly outlined in Chapter 1, the Mössbauer effect concerns the resonant absorption of a gamma ray, of frequency ω0 and wavevector k, by a nucleus. If the absorbing nucleus is located at the position r, the electromagnetic field of the gamma ray at the nucleus can be represented by

where ΓN is the natural linewidth of the decaying nucleus in its excited state, which is given by 2π/τN, where τN is the mean lifetime of the nuclear excited state. In general, in this chapter, linewidths are expressed in terms of frequency, which must be multiplied by ħ, the Planck constant divided by 2π, in order to convert to an energy.