SI-29 SUPERHYPERFINE INTERACTIONS OF THE E' CENTER - A POTENTIAL PROBE OF RANGE-II ORDER IN SILICA GLASS

An electron spin resonance study has been carried out on E'(gamma) centers in gamma-irradiated silica glasses of various Si-29 enrichments, with the object of measuring superhyperfine splittings due to Si-29 nuclei at the three closest silicon neighbors to the silicon of the unpaired spin. Experimental spectra obtained at X band (9.1 GHz) in the absorption mode and at Ka band (35.0 GHz) in the rapid-passage dispersion mode are compared with computer lineshape simulations based on a simple tight-binding model anchored to literature values of the so-called 'weak'Si-29 hyperfine splittings of the E'(1) center in alpha-quartz. Surprisingly few E'(gamma) sites are found in the glasses to exhibit such 'weak' splittings. To reconcile this result with the model assumption made by Mozzi and Warren and the supporting analyses of Galeener, it is suppose that dihedral angles in grassy silica might be relatively uniformly distributed and uncorrelated with the Si-O-Si angle distribution in the undamaged glass but that backward puckering relaxations (similar to that calculated by Rudra et al. for the E'(2) center in quartz) may be pervasive at E'(gamma) (oxygen vacancy) sites in glassy silica, causing dihedral angles in the 100 less than or similar to alpha(1) less than or similar to 140 degrees to be 'read out' in the present experiment as failing in the range 80 greater than or similar to alpha(1) greater than or similar to 40 degrees