Electron paramagnetic resonance, optical absorption, and magnetic circular dichroism studies of the CO 3 ? molecular-ion in irradiated natural beryl

Abstract

Room temperature X-irradiation of some natural beryls produced several new absorption lines in the electron paramagnetic resonance (EPR) spectrum, a known series of optical absorption lines in the 500–700 nm range, and a shift of the absorption edge to lower energies. Several of the new EPR lines and part of the irradiation-induced shift of the absorption edge disappeared after a few days at room temperature, and were not examined in detail. However, three of the paramagnetic centres responsible for the new EPR lines were stable at room temperature and two of these have previously been identified as atomic hydrogen and the methyl radical, CH3. These species were stable to ∼150 and ∼450°C respectively. The third stable species, hitherto unreported, showed a single-line EPR spectrum of axial symmetry, with g∥=2.0051 and g⊥=2.0152. This spectrum was found to be intensity-correlated with the series of optical bands in the 500–700 nm range, after thermal bleaching at 175°C. The EPR and optical spectra are therefore assigned to the same species. It is argued that this species is the CO 3 − molecular ion, located in the widest part of the structural channel and aligned with the plane of the molecule perpendicular to the c axis. The EPR spectrum is consistent with a 2 A′2 ground state of a CO 3 − molecule with trigonal symmetry, and this requires that the optical transition has a 2 A′2 → 2 E′ character. Most of the features in the optical spectrum can be assigned to coupling of a totally symmetric mode of frequency ∼1020 cm−1 onto a zero-phonon line at 14,490 cm−1 and a second weaker line at 16,020 cm−1. However, both of these two fundamental lines are structured, and the two components show strong temperature-dependent derivative-shaped magnetic circular dichroism (MCD). Furthermore, the overall sign of the MCD for the line at 16,020 cm−1 is opposite to that at 14,490 cm−1. The separation (∼120 cm−1) of the two components of the 14,490 cm−1 line is much larger than that expected from spin-orbit interaction, and the origin of this splitting is not yet understood.