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. Mossbauer emission spectroscopy of Co-57-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 the Co-57(EC)Fe-57 decay, low energy excitations of crystal field and Zeeman states, linkage isomerism, radical formation with subsequent redox reactions, and others. We have extensively studied Co-57-labelled complexes with [Co(II)N6] 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 with 1A1 ground state) and in complexes of intermediate ligand field strength (where the corresponding iron(II) compounds exhibit thermal spin-crossover 1A1 half-arrow-right-over-half-arrow-left 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. In Co-57-labelled LiNbO3 and other matrices, we have observed non-thermalized 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.