Luminescence of Cr2+ Centres and Related Optical Interactions Involving Crystal Field Levels of Chromium Ions in Zinc Sulfide

Abstract

In the i.r. emission of ZnS:Cr crystals, a zero-phonon doublet at 5211 and 5216 cm-1 and a number of vibronie bands are resolved at T ≈ 4 K. The Td symmetry of the crystal field at the Cr2+ ion on a Zn lattice site is reduced to D2d by a static Jahn-Teller effect. Differences in the vibronie structures of absorption and emission indicate prevalence of quasi-local vibrations in the spectra. An exponential decay of the i.r. luminescence in the μs range yields an oscillator strength f = 6×10-4 (T = 4K) in accordance with selection rule reasoning for the radiative transition 5A1(5E)→5B2(5T2). By interrelating a fit of transmission spectra with excitation spectra of luminescence and photoconduction, the distance of the Cr2+ centres from the conduction band edge is evaluated at 20700 cm-1 (T = 300 K). An irradiation of the crystals with photons of energies above this optical ionization threshold causes a sensitization for the Cr2+ i.r. luminescence. Thus, apart from effects related to traps, a novel excitation band at 10500 cm-1 arises which is interpreted by the 6A1(6S)→4T1(4G) transition of Cr+ centres. This transition terminates in a level degenerate with conduction band states so that free carriers will be released which, in turn, induce the Cr2+ emission. Furthermore, at some of the crystals, the i.r. emission between 2500 and 3000 cm-1 of Fe2+ centres is recorded with the best resolution obtained so far.