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O. Diwald, M. Sterrer, E. Knözinger, P. Sushko, A. Shluger:
"Wavelength selective excitation of surface anions on highly dispersed MgO";
Journal of Chemical Physics, 116 (2002), S. 1707 - 1712.

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 ion 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.

http://ojps.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JCPSA6000116000004001707000001&idtype=cvips