Infrared Luminescence ofFe2+in ZnS

Abstract

The infrared luminescence spectra of four different crystals of cubic ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{S}$ with ${10}^{\ensuremath{-}4}\ensuremath{\le}X\ensuremath{\le}{10}^{\ensuremath{-}2}$ have been measured at 5 and 77\ifmmode^\circ\else\textdegree\fi{}K. There is no appreciable luminescence at 300\ifmmode^\circ\else\textdegree\fi{}K. The luminescence of the ${\mathrm{Fe}}^{2+}$ iron occurs in a band between 2400 and 3000 ${\mathrm{cm}}^{\ensuremath{-}1}$ (4.2 to 3.3 \ensuremath{\mu}), and shows pronounced zero-phonon lines at 5\ifmmode^\circ\else\textdegree\fi{}K for the $^{5}T_{2}$-to-$^{5}E$ transition. Four of the five levels of the $^{5}E$ state can be seen, and their interlevel spacing is 18\ifmmode\pm\else\textpm\fi{}4 ${\mathrm{cm}}^{\ensuremath{-}1}$. Several phonon-assisted transitions are also present, the involve single TA, TO, or LO phonons. In the purest crystal the zero-phonon component accounts for 50% of the luminescent intensity. The luminescence spectrum is not a mirror image of the absorption spectrum, which has a much smaller zero-phonon component. The luminescence output is reasonably independent of the ${\mathrm{Fe}}^{2+}$ concentration, thus indicating that iron is not a "killer" of its own luminescence.