Surface colour centers on magnesium oxide doped by magnesium and alkali metal vapours

The addition of small amounts of low ionization energy metals to the surface of magnesium oxide has been studied by EPR spectroscopy. The metals investigated are magnesium and the alkali metals from lithium to rubidium. In the experimental conditions adopted the added metal atoms are ionized and the electrons trapped at the surface in the form of both one-electron (F,+) and two-electron (F,) colour centres. The F,+ paramagnetic centres formed are similar to those centres obtained by y- or UV-irradiation of the solid. The linewidth and saturation properties of the EPR spectra depend on the nature of the added metal, even though the nature of the interaction between the released electron and the parent cation is still unclear. A less intense line present in all spectra is attributed to subsurface F+ centres.