Exciton and Interband Spectra of Crystalline CaO

Absolute, near-normal-incidence reflectance spectra were obtained for CaO at 300°K from 6 to 35 eV and at 80 and 25°K from 6 to 12 eV. The important optical parameters were obtained by Kramers-Kronig inversion of the reflectance spectra. The ${\epsilon }_2$ spectrum of CaO at 25°K showed two strong-exciton peaks at 6.79 and 11.42 eV, a set of weak-exciton doublets, and strong-interband peaks at 10.0, 12.1, and 16.9 eV. Interband structure observed in the ${\epsilon }_2$ spectrum of CaO was associated with specific transitions by analogy with the band structure of MgO. The main features of the resulting band structure were: (1) The first conduction band of CaO is about 1 eV lower than in MgO; (2) the $X_3$ point of the second conduction band is lowered by about 7 eV from that in MgO.