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 O⎯3 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 O2O2 pressures molecular oxygen does not only act as an electron trap, favoring the O⎯ formation, but it also contributes to UV induced O⎯3 formation with a maximum efficiency at 4.2 eV.
Diwald, O., Sterrer, M., Knözinger, E., Sushko, P.V. and Shluger, A.L., 2002. Wavelength selective excitation of surface oxygen anions on highly dispersed MgO. The Journal of chemical physics, 116(4), pp.1707-1712.
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