Mössbauer emission spectroscopy of metastable electronic states

In⁵⁷Fe Mössbauer emission spectroscopy, the parent nucleus⁵⁷Co is incorporated in the matrix by diffusion, implantation or co-precipitation. Mössbauer emisssion spectra often differ from the corresponding absorption spectra in several aspects. The “anomalous” features (denoted as “after-effects” of nuclear transformation) comprise mainly (a) anomalous charge states, (b) anomalous spin states, and (c) metastable electronic populations within these charge states. The first two classes of effects were extensively studied during the sixties and seventies. More recently, several investigations have shown that, under certain circumstances, the electronic systems of Fe(II) and Fe(III) have not reached the thermal equilibrium within the Mössbauer time window. These effects have been evidenced (i) within the crystal field ground-state manifolds, but also (ii) as spectral contributions from excited crystal field states. In the present paper, an overview of these experiments will be given, together with their theoretical interpretation and in comparison to results obtained with other methods (e.g. optical spectroscopy). Special emphasis will be given to the systems⁵⁷Co/LiNbO₃ and⁵⁷Co/MgO, where nonequilibrium populations within the⁶A₁ ground manifold of Fe(III) and Γ_5g ground manifold of Fe(II) have been observed in an external magnetic field.