Wavelength selective excitation of surface oxygen anions on highly dispersed MgO

Monochromatic UV light in the spectral interval between 4.0 and 5.5 eV is used in order to selectively excite 3- and 4-coordinated oxygen anion sites on the surface of MgO nanoparticles exposed to O2 gas. As a result, two different paramagnetic O− surface species and also ozonide anions O3− are observed by electron paramagnetic resonance (EPR) spectroscopy. The relative abundance of each of the O− species exhibits a specific dependence on the energy of the exciting photons. EPR data together with the results of theoretical modeling suggest that both O− species are located at 3-coordinated sites having different local environments. At sufficiently high O2 pressures molecular oxygen does not only act as an electron trap, favoring the O− formation, but it also contributes to UV induced O3− formation with a maximum efficiency at 4.2 eV.