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

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 r « 4 K. The Td symmetry of the crystal field at the Cr2+ ion on a Zn lattice site is reduced to J>2(1 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 us range yields an oscillator strength y = 6X10~4 (J = 4K) in accordance with selection rule reasoning for the radiative transition 5At (5E) —5B2 (5To) . 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 (7 = 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 6At (6S) —>• 4Tt (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.