Electron spin resonance studies on hydrogen atoms formed in pure and acidic ices under electron irradiation. Motional narrowing and electron spin polarization effect

Abstract

Hydrogen atoms formed in pure ice and in frozen solutions of sulfuric and hydrochloric acid were observed by ESR spectroscopy during continuous electron irradiation. In the sulfuric acid ice a large decrease in the hydrogen atom line width occurred around -130/sup 0/C. Above -110/sup 0/C an anomaly which is characteristic of the spin polarization effect was concurrently observed with further gradual narrowing of the line width. While no hydrogen atom was detected in pure ice below -130/sup 0/C, a narrowed signal emerged above -110/sup 0/C. This also showed the polarization effect. The narrowing was attributed to diffusional motion of hydrogen atoms, and was analyzed with a simple classical method assuming an exponential correlation function for the fluctuating field. The narrowing around -130/sup 0/C in sulfuric acid ice is related to an escape process from the solute trap, while above -110/sup 0/C both acidic and pure ices seem to involve a similar diffusional process with a low activation energy. The polarization of hydrogen atoms observed was distinct from the reported result in the liquid phase in that the low-field line can sometimes be more strongly polarized than the high-field line. This point was discussed in terms of the radical pair theory,more » taking account of the relatively slow diffusion of hydrogen atoms in solid ice.« less