Metastable electronic states induced by nuclear decay and light

Radioactive atoms incorporated in insulating solid-state compounds create various kinds of chemical and physical after-effects upon nuclear disintegration. Mössbauer 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 Co^IIN₆] cores employing time-integral and time-resolved Mössbauer 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.