AN ELECTRON SPIN RESONANCE STUDY OF RADIATION-INDUCED CENTERS OF CHEMICAL ACTIVITY IN A SILICA-ALUMINA GEL

The esr spectrum of a γ-irradiated silica-alumina gel has a line 44.5 ± 0.5 G wide at g = 2.0088 ± 0.0005 which is partially resolved into six components separated by 8.5 ± 0.2 G; superimposed on this broad line is a narrow line at g = 2.0010 ± 0.0005. The broad line is identified with a positive hole trapped at an aluminum atom present substitutionally in the silica matrix. The narrow, line is attributed to an electron trapped at one of several kinds of oxygen vacancies and seems to result from a reaction of H atoms. An appreciable number of electrons are trapped in such a manner as to be undetectable in the esr spectrum. At large doses, the positive-hole intensity reaches a limiting value that corresponds to 16 × 1017 spins/g; such a value is comparable with a previously determined value of 13 × 1017 centers/g as the limiting yield of radiation-induced centers of chemical activity. On exposure of irradiated solid to each of 18 reagents and on irradiation in the presence of each of 12 reagents, without exception, behavior of the visible coloration parallels that of the broad esr signal. With H2 and H2O as the only exceptions, a correlation exists between ionization potential of a reagent and its reactivity with the trapped positive holes; such a correlation permits an estimate of the effective electron affinity of the positive hole as lying between 9.8 and 12.1 eV. When present during irradiation, O2, N2O, and CO2 act as traps for free electrons and enhance color and positive-hole signal of the solid; CO and SO2 apparently act as traps for free positive holes (unstabilized O-) to give CO2- and SO3-. A model is presented to describe the generality of results obtained in radiation studies with silica-alumina gel.