Electron spin resonance absorption spectrum of HCO 32− molecule-ions in irradiated single-crystal calcite

Abstract

ESR and ENDOR spectra of γ-irradiated naturally occurring single-crystal calcite have revealed the existence of a stable and hitherto undetected molecule-ion defect associated with hydrogen The ESR absorption spectrum consists of six hyperfine structure doublets arising from paramagnetic centers with effective spin S= l 2 and I= 1 2 at two sets of three equivalent lattice sites From the mean of the principal values of the orthorhombic g tensor as well as from the intensity and magnitude of 13C self-hyperfine structure and superhyperfine structure, the center appears to be similar to the CO 3 3− molecule-ion. ENDOR data identify the associated nucleus to be 1H. Analysis of the isotropic and anisotropic contributions to the orthorhombic 1H hyperfine structure tensor indicates that about 2.5 % of the unpaired electron spin density is localized on the proton. From this and other evidence obtained from temperature-dependent linewidth studies over the range 200 to 300°K, it is suggested that the proton is a constituent of the paramagnetic molecule-ion. It is proposed that the defect center is HCO 3 2− and that it arises from the ionization of bicarbonate ion impurities in calcite.