The fundamental optical absorption in the range of $h\nu$ from 2.5 to 5.8 ev has been measured on well-annealed thin films of the following compounds: BaO, SrO, BaS, SrS, CaS, BaSe, SrSe, CaSe, MgSe, BaTe, SrTe, CaTe, and MgTe. The absorption peaks are ascribed to excitons and are observed to shift to lower energies for increasing atomic numbers of either the IIA or VIB elements. A doublet splitting is observed which is close to the values for the spin-orbit splitting of the free negative VIB ions for the selenides and tellurides but which is somewhat larger for the heavier oxides and sulfides.

• Received 21 October 1958

DOI:https://doi.org/10.1103/PhysRev.113.1019