Isotope effects on exciton energies in CdS

The energies of free and bound excitons related to the $A$ and $B$ valence bands in isotopically pure wurtzite CdS ${(}^{110}$CdS, ${}^{112}$CdS, ${}^{114}$CdS, ${}^{116}$CdS) and natural CdS have been measured at low temperature by means of photoluminescence, piezomodulated reflectivity, and photomodulated reflectivity. With an increase of the Cd mass the excitonic energies increase by about 40–68 $\mu$eV/amu, one order of magnitude less than observed when increasing the S mass. These results can be understood from the point of view of electron-phonon interaction, provided one takes into account the 4$d$ electrons of Cd that hybridize in the valence band with the 3$p$ states of sulfur. The results are compared with existing experimental data of the temperature-dependent excitonic energies in CdS.