Optical Absorption and Photoconductivity in Barium Oxide

Measurements of the optical absorption and photoconductivity in BaO single crystals are reported. The threshold photon energy for both processes is 3.8 ev, and the absorption constant at higher energies is at least 10⁵ cm^-1. A second increase in the absorption constant begins at about 4.8 ev and is not accompanied by photoconductivity. The observed dependence of photoconductivity on temperature and electric field strength indicates space charge effects in limiting current flow. The onset of absorption at 3.8 ev is thought to be due to the production of excitons, with the accompanying photoconductivity due to thermal dissociation of excitons or exciton ionization of impurity centers.