Mössbauer absorption and emission study of diluteFe2+impurities in cubic ZnS. Observation of metastable electronic levels. I. Relaxation measurements

In dilute ZnS:${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ absorbers the isolated ${\mathrm{Fe}}^{2+}$ ions give a single Mössbauer absorption line at all temperatures ($1.4<~T<~295 \mathrm{K}$), but a relaxation broadening is observed near 8 K. A convenient relaxation line-shape theory has enabled the transition rate $W_{{\Gamma }_4\to {\Gamma }_1}$ from the first excited spin-orbit triplet ${\Gamma }_4$ to the ground-state level ${\Gamma }_1$ to be measured between 6 and 13 K. Two additional ${\mathrm{Fe}}^{2+}$ quadrupole doublets appear for $T\lesssim 5$ K in the ZnS:${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ emission spectra. These are due to the slow relaxation contributions of the excited spin-orbit triplets ${\Gamma }_4$ and ${\Gamma }_5$ of ${\mathrm{Fe}}^{2+}$ which are then observed out of the thermal equilibrium following the decay of the radioactive parent ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$. The two transition rates $W_{{\Gamma }_4\to {\Gamma }_1}$ and $W_{{\Gamma }_5\to {\Gamma }_1}$ were obtained by following the intensities of the ${\Gamma }_4$ and ${\Gamma }_5$ lines in the emission spectra below 5 K when these rates are comparable to the inverse nuclear lifetime $\frac{1}{\tau }$. Both the absorber and source measurements of $W_{{\Gamma }_4\to {\Gamma }_1}$ show approximately a $T^{5.0}$ mean thermal variation, whereas the source measurements of $W_{{\Gamma }_5\to {\Gamma }_1}$ present a $T^{4.0}$ mean variation. These relaxation rates are attributed to Raman processes, which have been shown to follow the thermal dependence