Nuclear level mixing-induced interference in FeCO3

We present a detailed description and a consistent explanation of the transparency observed in the Moessbauer absorption spectra at a nuclear level mixing in FeCO3. We develop a model for scattering in this nuclear Λ system in a Maxwell-Schroedinger approach, taking into account multiple scattering and polarization effects. It is shown that the level mixing scheme not only yields an expected Stark splitting, but, due to the unequal relaxation rates and transition strengths, an additional interference term is present. Therefore, this scheme is equivalent to the Λ scheme considered for EIT in quantum optics. The interference in the absorption of the gamma radiation can be destructive, leading to a partial transparency, but can also be constructive. Both types of interference are present in the observed spectra.