The interaction of na atoms with the surface of alkaline-earth oxides. Possible implications for a "Magnetic basicity" scale

The magnetic properties of Na atoms deposited on the surface of alkaline-earth oxides (MgO, CaO, SrO) are studied by means of electron paramagnetic resonance (EPR) spectroscopy. In all cases, isolated monomeric species are observed, which are characterized by a four-line spectrum because of the Na-23 hyperfine interaction. This interaction is found to markedly depend upon the basicity of the oxide decreasing from MgO (14 mT) to SrO (7.4 mT). This behavior is interpreted in terms of strong perturbations of the Na 3s orbital induced by the O2- surface anions and is discussed in the framework of an expanded orbital approach. A linear correlation between the Na-23 hyperfine coupling constant (a(iso)), the optical basicity (Lambda), and the Madelung potential of the oxides is found, suggesting that the spin density of the alkali metal may be used as an effective probe of the surface basicity.