Fundamental Absorption of Barium Oxide from Its Reflectivity Spectrum

The reflectivity spectrum of single crystals of barium oxide, cleaved in vacuum, was measured as a function of temperature. The Kramers-Kronig integral relation was applied to derive the spectral dependence of both the absorption constant and index of refraction in the fundamental absorption region. The absorption compares favorably with results of previous thin-film transmission measurements and confirms the intrinsic nature of the multiple structure, recently explained by Overhauser as resulting from spin-orbit interaction in exciton creation. Large time-dependent changes of the reflectivity spectrum were observed even in a vacuum of 10^-7 mm Hg at liquid-nitrogen temperature and are interpreted as interference effects from a growing surface layer of Ba (OH) ₂, formed with a diffusion-limited rate of growth.