After effects following nuclear transformation have been extensively studied in a large variety of matrices by Mossbauer Emission Spectroscopy (MES). The branching ratios of transient atomic charge states and energetically unstable states of the environment, excitations of electronic ligand field states and populations out of equilibrium within the electronic ground state manifold have been studied. Recent developments and also new insights and understandings of different aspects of the aftereffects of radioactive decay of57Co in semiconductors and molecular crystals are reviewed. A detailed picture of the decay process within the electronic states of the nucleogenic Fe3+ species could be obtained from emission spectroscopy of sources in applied magnetic fields.

Advances in Mössbauer Emission Spectroscopy

Radioactive atoms incorporated in insulating solid-state compounds create various kinds of chemical and physical “after-effects” upon nuclear disintegration. Mossbauer emission spectroscopy of57Co-labelled coordination compounds has undoubtedly become the most informative technique to detect such after-effects like aliovalent charge and spin states of the nucleogenic iron atom resulting from the57Co(EC)57Fe decay, low energy excitations of crystal field and Zeeman states, linkage isomerism, radical formation with subsequent redox reactions, and others. We have extensively studied57Co-labelled complexes with [CoIIN6] cores employing time-integral and time-resolved Mossbauer emission spectroscopy. In complexes of strong ligands fields (where the corresponding synthesized iron(II) complexes show low spin behaviour with1A1 ground slate) and in complexes of intermediate ligand field strength (where the corresponding iron(II) compounds exhibit thermal spin-crossover1A1 ⇌5T2) we have observed the population of metastable high spin states, which is strongly time- and temperature-dependent in the case of the strong-field complexes. Irradiation of iron(II) spin-crossover complexes with light also induces the formation of metastable high spin states, which are the same in nature as those resulting from decaying nuclei as an “intramolecular light source”. The mechanisms for “light-induced excited spin state trapping” (LIESST) and “nuclear decay induced excited spin state trapping” (NIESST) have been elucidated; they are strongly related to each other. In57Co-labelled LiNbO3 and other matrices, we have observed nonthermalized populations of the Zeeman sublevels of the ligand field ground state. These low energy excitations and the metastable ligand field states constitute the last stages of the slowing down process after the nuclear decay.

Metastable electronic states induced by nuclear decay and light

Mössbauer absorption and emission study of iron impurity centers in ZnTe

We have performed Mossbauer measurements on oriented powders of 57Fe-doped La2−xSrxCuO4 (x=0 and 0.2 ) and 57Co-doped La2CuO4. We find that the main axis OZ of the 57Fe electric field gradient in the series lies at an angle β=26°±5° with respect to the crystal c axis, which is much less than recently published values. Our data were corrected for a fraction of unoriented crystallites, which we find to be systematically present, but which was unsuspected in previous experiments. We describe a convenient X-ray examination procedure for detecting and evaluating the unoriented fraction. We discuss possible origins of the angle β and we examine the relative disposition of the direction of c, OZ and of the hyperfine field on 57Fe in La2CuO 4 below TN. We also consider the influence of the oxygen stoichiometry on the symmetry at the Fe site.

57Fe Mössbauer absorption and emission spectroscopy and X-ray measurements in oriented crystallites of La2−xSrxCuO4

In57Fe Mossbauer emission spectroscopy, the parent nucleus57Co is incorporated in the matrix by diffusion, implantation or co-precipitation. Mossbauer 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 Mossbauer 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 Γ5g ground manifold of Fe(II) have been observed in an external magnetic field.

Mössbauer emission spectroscopy of metastable electronic states

The entire Mossbauer absorption spectrum of 57 Fe impurities in CaF 2 and the main part of the emission spectrum of a CaF 2 ( 57 Co) powder sample originate from substitutional Fe 2+ ions in cubic sites A weak Fe 1+ (3d 7 ) charge state contribution is also detected in the emission spectra, but no Fe 3+ contribution is observed Two long-lived excited electronic level contributions are evidenced out of the thermal equilibrium in the low temperature emission spectra of the cubic site Fe 2+ ions The first originates from the low energy spin-orbit triplet 5 E-Γ 4 (E≃ 16 cm −1 ), whose relaxation properties are also analysed by absorption spectroscopy, and the second probably originates from the highly excited level 5 T 2 -Γ 5g (E≃ 5000 cm −1 ) Observation de Fe 2+ en site cubique dans les spectres d'absorption et d'emission et d'une faible contribution de Fe + dans les spectres d'emission Pour Fe 2+ a basse temperature, mise en evidence de deux contributions issues de niveaux electroniques excites, a longue duree de vie et peuples hors equilibre thermique: 1) triplet spin-orbite 5 E-Γ 4 de faible energie (E∼16 cm −1 ) et 2) niveau excite 5 T 2 -Γ 5g de haute energie (E∼5000 cm −1 )

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Mössbauer absorption and emission experiments in CaF2(57Fe): relaxation and after-effect study

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Mössbauer absorption and emission experiments in CaF2(57Fe): relaxation and after-effect study