Spectroscopic properties of trapped electrons on the surface of MgO nanoparticles

To characterise electron-trapping sites on the surface of MgO nanoparticles, surface colour centres were generated using UV light in conjunction with selected hydrogen-based electron sources. Four different colour-centre species, including the characteristic (e(-))(H+) or F-s(+)(H) centre, were identified due the othe distinct shape of the respective elctron paramagnetic resonance (EPR) signals. The analysis of the EPR saturation behaviour down to microwave power of 5 X 10(-3) mW reveals an enhanced spin-relaxation probability of the (e(-))(H+) centre compared to all other F-s(+) centres that do not exhibit significant magnetic interactions with hydroxylic protons. Beside the dipolar magnetic interaction in the (e(-))(H+) centre observed by EPR, the electronic interaction between the unpaired electron and the proton of a closely spaces OH group produces a redshift of the OH stretching band by about 70 to 170cm(-1), as observed by infrared spectroscopy EPR and IR spectroscopic data obtained after the selective address of individual reaction channels for surface colour-centre formation point the to the fact that (e(-))(H+) centres are formed by trapping electrons from H atoms. Consequently the underlying surface defect does not belong to the sites of the MgO surface, which chemisorb hydrogen via a heterolytic splitting process.