Donor-Acceptor Pair Lines in Cadmium Sulfide

We report the first observation of donor-acceptor pair lines in CdS. The spectrum correlates with a distant pair peak at 5176 Å, an $I_1$ line at 4888.52 Å, and an $I_2$ line at 4869.30 Å. Several other pair-line spectra were observed in the vapor-grown undoped platelets used in these experiments. One of these correlated with the same $I_1$ line and distant pair peak, but with an $I_2$ line at 4869.17 Å. Zeeman experiments confirmed that these lines were donor-acceptor pair lines. The Zeeman pattern of the pair lines could be predicted, using a crystal field and a $j-j$ coupling constant. These constants were determined by the splittings of the pair lines in zero field. For $\mathbf{H}\parallel c$, the hole $g$ value was the same as for an isolated acceptor. A theoretical wurtzite spectrum was calculated on the basis of Coulomb interactions. The spectrum was much more complicated than a theoretical zinc-blende spectrum and was sensitive to both the dielectric-constant ratio $\frac{{\epsilon }_{\perp }}{{\epsilon }_{\mathrm{II}}}$ and the $\frac{c}{a}$ ratio of the CdS lattice. The experimental lines could not be assigned, but the line densities were in agreement with what was expected for a pair-line spectrum involving a simple donor and acceptor.