Oxygen, adsorbed anion radical formation

Fig. 6. Evidence from ESR for the growth of oxygen anion radicals at gas/solid interfaces exposed to various radiations, (a) (i) Kinetic curves for chemisorption of oxygen (1) and the formation of ion radicals 02 (2) on A1203 under the action of 7-irradiation. T = 25°C, dose rate = 0.5 X 1017 eV g-i min (ii) ESR spectrum of 02 adsorbed on 7-Al203. Reproduced with permission and minor adaptation from ref. 81. (b) Changes in intensity of ESR signal of 02 at g 2.0, /2 0, and in oxygen pressure on illumination of an 02/Zr02 sample. Reproduced with permission and minor adaptation from ref. 82. (c)(i) ESR spectrum of O and (ii) its growth at an N20/ZnO interface on exposure to UV illumination. Reproduced with permission from ref. 85.

It is also possible to form radical cations and radical anions on the same alumina or silica-alumina surface (88). One of the more interesting observations was that a marked enhancement of the radical anion spectrum for trinitrobenzene results when perylene is adsorbed on an alumina surface, and similarly the radical cation signal is reenforced by adsorption of trinitrobenzene. The linewidths of the spectra confirm that the radical ions are separated by a distance greater than 10 A. This means that the electron must be transfered through the lattice or that the ions separate after the transfer step, which seems unlikely. Oxygen was still required for the formation of the radical cation. 

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