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, τ_M≈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.