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 T d symmetry of the crystal field at the Cr 2+ ion on a Zn lattice site is reduced to D 2d 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 5 A 1 ( 5 E)→ 5 B 2 ( 5 T 2 ). By interrelating a fit of transmission spectra with excitation spectra of luminescence and photoconduction, the distance of the Cr 2+ 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 Cr 2+ i.r. luminescence. Thus, apart from effects related to traps, a novel excitation band at 10500 cm -1 arises which is interpreted by the 6 A 1 ( 6 S)→ 4 T 1 ( 4 G) 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 Cr 2+ emission. Furthermore, at some of the crystals, the i.r. emission between 2500 and 3000 cm -1 of Fe 2+ centres is recorded with the best resolution obtained so far.