It was noted that the thermochemical reduction of H-doped single crystals of these oxides resulted in the creation of high concentrations of H- ions. In the case of MgO, positron lifetime and Doppler broadening data showed that positrons were trapped at H- centers and formed positronium hydride molecules, [e+-H-]. A value of 640ps was found for the lifetime of positronium hydride states that were located in an anion vacancy. Positrons were also trapped at H2- sites in MgO at low temperatures. The H2- ions were introduced by illumination with blue light. The formation of positronium hydride states in CaO could not be firmly established. It was found that, after heating MgO (1470K, 2h, reducing atmosphere), anion vacancies completely vanished. Under stimulation with blue light, the H2- concentration was estimated to be at least an order of magnitude lower than that of the H- ion. However, the H2- ion was expected to be a more effective positron trap because of its more negative potential. The S-values under 400nm light stimulation were lower than those measured in the dark at temperatures below 100K. At higher temperatures, the S-values which were measured in the dark and those which were measured under illumination were indistinguishable. A low S-value implied that positron annihilation with low-momentum electrons was less probable when the samples were illuminated with 400nm light. The dark/light difference was attributed to the photo-conversion of H- ions into H2+ ions by the capture of electrons that were released from Fe+ impurities. The observed results were explained in terms of a reduction in the creation of positronium hydride-like states, due to the photo-conversion, H-  H2-, and the subsequent formation of bound [e+-H2-] states. Annihilating e--e+ pairs at [e+-H2-] states had lower momenta than pairs which annihilated at positronium hydride states.

M.A.Monge, R.Pareja, R.González, Y.Chen: Materials Science Forum, 1997, 239-241, 47-50