Γ-Point Valence-Band Energy Levels in CdS Determined from Excited States ofA- andB-Band Excitons

Thomas and Hopfield measured the exciton absorption spectrum of CdS in 1961 and identified several of the intrinsic exciton states. The present paper reports the $n=2\mathrm{}$, $n=3\mathrm{}$, and $n=4\mathrm{}$ excited states of intrinsic excitons, whose holes derive from the $B$ valence band, as observed in emission spectra of single-crystal CdS platelets at 1.2°K. The energy difference between the $1S$ ${\Gamma }_{5T}$ ground state and the $2S$ state of the $B$-band exciton is less than the energy differences between corresponding states of the $A$-band exciton by approximately 4 meV. That this deviates from the predicted effective-mass hydrogenic-exciton approximation can be explained by taking into account the details of the valence bands. The binding energies of the $A$- and $B$-band excitons, calculated from the higher excited states, are the same within experimental error in the hydrogenic approximation.