Time-dependent perturbation of Mössbauer spectra

In this paper we derive a complete semiclassical description of Mössbauer spectroscopy with arbitrary external or internal perturbations of the active levels. The γ rays are taken to be weak enough to allow us to evaluate the linear Mössbauer response to lowest order in the intensity, in which case the γ-ray field can be treated classically. It induces transitions between level manifolds that can have arbitrary interactions inside themselves. The nuclear system is described by a density matrix. In principle, time-dependent perturbations can be handled by the formalism, but the equations are easier to solve for systems that display steady-state behavior. We consider, in particular, the case of magnetic radio-frequency modulation of the Mössbauer levels. Utilizing a matrix-continued-fraction technique, we solve for the absorption spectrum, and we choose numerical illustrations with parameters such that they describe the nucleus ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$. In this paper we treat mainly the case of magnetic modulation, but other experimental arrangements are discussed too. The calculations are able to treat both saturation and interference effects, which become important when the radio-frequency modulation is very strong. The new feature that emerges is the occurrence of resonances arising from the interference of different physical processes. The calculational method and its results are discussed and compared with other works.