Item

Abstract

New structured luminescence bands have been observed near 4400 cm−1 with CdS and near 4200 cm−1 with CdSe crystals at cryo-temperatures. These emissions are identified as the internal 5E(D)→5T2(D) translations of the Cr2+(D4) ions. For CdS: Cr the observed luminescence displays a weak zero-phonon doublet at 4680; 4674 cm−1. The symmetry of the CrCr2+ centre in the ground state 5T2(D) being reduced to D2d by a nearly static Jahn-Teller effect, the doublet can be assigned to Γ3,Γ4-5Â1(E)→Γ5/Γ1,Γ2-5B̂ 2(T2)transitions between spin components. The excitation spectra of the internal emission display intense charge-transfer bands at energies above 12,000 cm−1 and 10,000 cm−1 with CdS and CdSe, respectively. These bands correspond to donor-type and in response to an conduction band. Indications of acceptor-type processes have been found in the excitation spectra and in their response to an auxiliary irradiation. Some narrow peaks represent the bound and free exciton regimes. With CdS: Cr2+, an exponential decay of the luminescence is recorded, characterised by a time constant τ=8.1 μs.