Electron-spin-resonance identification of a -CH2 -dot radical in irradiated amorphous SiO2:OH.

Abstract

A three-line electron-spin-resonance (ESR) spectrum has been observed following x irradiation in three of four high purity amorphous silica samples independently manufactured via the flame hydrolysis method (type-III silica). This spectrum, with a line separation of approximately 18 G, was previously attributed to an unpaired spin undergoing a hyperfine interaction with nitrogen. Optimization of ESR parameters and irradiation procedures have been employed in the present work to obtain spectra of unprecedented signal quality. A direct correlation of the three-line ESR signal with the presence or absence of two carbon radicals, HC${\mathrm{O}}^{\mathrm{\ifmmode\cdot\else\textperiodcentered\fi{}}}$ and C${\mathrm{H}}_{3}^{\mathrm{\ifmmode\cdot\else\textperiodcentered\fi{}}}$, in the four different silica samples is observed. Studies of defect concentration vs x-ray dose at room temperature show that the three-line defect forms concurrently with the decay of HC${\mathrm{O}}^{\mathrm{\ifmmode\cdot\else\textperiodcentered\fi{}}}$, and prior to the appearance of C${\mathrm{H}}_{3}^{\mathrm{\ifmmode\cdot\else\textperiodcentered\fi{}}}$. Absolute spin counts are consistent with the evolution of all three defects from a single trace impurity. It is proposed that the three-line spectrum results from a hyperfine splitting due to the nuclear spins of two equivalent $^{1}\mathrm{H}$ ($I=\frac{1}{2}$), not $^{14}\mathrm{N}$ ($I=1$). Simulation of the experimental line shape gives excellent agreement with the two hydrogen model. The magnitude of the hyperfine splitting of this extrinsic silica defect is nearly the same as that due to the two equivalent hydrogens of the $^{\mathrm{\ifmmode\cdot\else\textperiodcentered\fi{}}}\mathrm{C}$${\mathrm{H}}_{2}$OH radical in methanol. Thus the three-line defect is identified as a -C${\mathrm{H}}_{2}^{\mathrm{\ifmmode\cdot\else\textperiodcentered\fi{}}}$ radical, not a nitrogen defect as had been previously supposed.