Nuclear Decay Induced Excited Spin State Trapping (NIESST)

Abstract

Mössbauer Emission Spectroscopy (MES) has been employed in studies of chemical and physical after-effects of the electron capture process 57Co(EC)57Fe in inorganic compounds. The 57Co labelled compounds are used as the Mössbauer source at variable temperatures vs a single-line absorber such as K4[Fe(CN)6]·3H2O kept at room temperature. The recorded ME spectrum yields information on the electronic and molecular structure of species containing the nucleogenic 57Fe in its first nuclear excited state (14.4 keV, τM≈140 ns lifetime). The after-effects observed in this time window refer to changes of charge and spin state, radiolysis products, metal-ligand bond rupture, long-lived metastable spin states and low energy excitations in spin-orbit coupling or Zeeman manifolds. Long-lived metastable spin states of 57Fe(II) have been observed in time-integral ME spectra of 57Co labelled coordination compounds, the corresponding iron(II) compounds of which are classified as strong-field and intermediate field (spin crossover) compounds. Their lifetimes have been measured using a MES coincidence spectrometer and found, at comparable temperatures, to be very similar to those observed of metastable Fe(II)-HS states after laser excitation (LIESST). The so-called inverse energy gap law, first introduced by Buhks et al. and later applied by A. Hauser to describe the relaxation of metastable LIESST states, also applies to the relaxation of metastable spin states generated by “nuclear decay-induced excited spin state trapping (NIESST)”. We therefore conclude that both phenomena, LIESST after optical excitation and NIESST after nuclear decay, follow the same relaxation mechanism.