Physical and chemical after-effects of nuclear decay in inorganic solids

Studying the chemical and physical after-effects of nuclear decay in inorganic (and organic) solids has been a field of particular interest in „Hot Atom Chemistry” ever since the discovery of recoil-labelling by Szilard and Chalmers in 1934. Most of the published mechanistic studies, however, suffered from the disadvantage that the researchers used standard wet-analytical methods for the determination of the labelled species. It is clear that metastable species, often being very important in the construction of reaction pathways, do not survive when the crystal is dissolved. We began in the late sixties using Mössbauer emission spectroscopy (MES) to study the metastable crystal field states arising from the 57Co(EC)57Fe nuclear decay in 57Co-doped coordination compounds with nitrogen donating ligands of different field strengths. The 57Co-doped compounds are employed as the Mössbauer source versus a single-line absorber such as K4[Fe(CN)6].

In strong-field compounds such as tris-phen complexes we observed for the first time metastable HS states of the nucleogenic 57Fe(II) ion /15/, which are seen in the emission spectrum only below ca. 200 K with increasing intensity upon lowering the temperature. Similar observations were made later in other strong-field diimine complexes /41/. In 57Co-doped complexes with weaker ligand fields like those leading to thermal spin transition in the corresponding iron(II) complexes, we have observed 57Fe(II) decay species only in the HS states, even at very low temperatures where the corresponding iron(II) complex is after spin transition in the LS state /36, 39, 42, 56/. With a special home-made coincidence spectrometer we were able to measure the lifetimes of the metastable HS states in strong-field complexes as function of temperature /43/. This technique, abbreviated as TDMES (Time-Differential Möss-bauer Emission Spectroscopy) was later improved several times /106, 116, 144/, and the Mainz TDMES apparatus is up to now the only one available. We can use it for relaxation studies of nuclear after-effects in the range of ca. 5 - 500 ns with a time resolution of ca. 3.5 ns. More recent TDMES measurements on the tris-phen complex /168/ and on a single crystal of the tris-bipy complex yielded, within experimental errors, the same lifetimes as those from optical relaxation studies /202, 241/. This agreement lends support to the conclusion that the mechanism of the formation of the metastable HS states following the nuclear decay as a molecular source of excitation is the same as that for LIESST after irradiation with an external light source.

MES measurements have also proven to be an elegant tool for probing local ligand field strengths. It is found that local pressure exerted by tight lattice surroundings increases the probability of observing the nucleogenic 57Fe(II) ion in the HS state as is the case with encapsulated 57Co-complexes in zeolites/269, 270/ or with 57Co-labelled terpy complexes with anions of different sizes /311/. Highly acknowledged by solid state physicists was also the ob-servation of non-thermalized excitations of Zeeman levels in LiNbO3 by MES /96/.

All these results from time-integral and time-differential MES measurements on inorganic solids have been summarized in extended review articles /88, 157,167,170,193, 388/.