Cross relaxation and radiative recombination of Co2plus ions in ZnS.

We report on high resolution Fourier transform spectroscopy of the absorbance, photoluminescence and photoluminescence excitation of the 4T2 - 4A2 transition of Co2+ in structurally perfect ZnS crystals of cubic, hexagonal and the 4H polytypic modification. Direct insight into the de-excitation mechanism of photo-excited centres has been found by comparing the intensities of the transitions in structurally pure 4H polytype ZnS: As expected, the absorbance strengths of the equally abundant quasi-cubic and quasi-hexagonal centres are identical. In contrast, the photoluminescence intensity of the quasi-hexagonal Co2+ centre is enhanced by a factor of nine compared to the quasi-cubic one. This is explained by an energy transfer from the quasi-cubic centres to the quasi-hexagonal cobalt ions. The photoluminescence excitation spectra give direct and quantitative evidence for this model.