Nearest-Neighbor Splitting of the Luminescence Levels ofZnSxSe1−x

The gradual shift of $\mathrm{Zn}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$ luminescence levels toward the valence band with decreasing $x$ is attributed to replacement of S by Se in the activator surround. If the center wave function is confined mainly to the four anions surrounding the activator, the center level should split into five levels corresponding to centers with surrounds containing $j$ S atoms and $4-j$ Se atoms. These five $j$ levels are not inconsistent with the gradual level shift toward the valence band. If the five levels are fitted to this shift, emissions from different $j$ levels are not resolved and combine to form a composite broadened band that shifts gradually with $x$. The broadening of the composite band agrees with that observed for the green bands for Ag, Cu, and self-activation. It also accounts for band crossovers through the terminal ZnSe band and for sliding sidewise emission shifts with quenching reported for Ag activation. The Ag blue band is narrower than calculated for five $j$ levels. Possible explanations are that the blue-center wave function extends to additional anions beyond the nearest neighbors or that the blue emission is partly absorbed at green centers.