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Dynamics of CO2− radiation defects in natural calcite studied by ESR, electron spin echo and electron spin relaxation

ESR spectra were recorded in the X-band (9.6 GHz) and in the W-band (94 GHz) and electron spin relaxation was measured by electron spin echo (ESE) in the temperature range 4.2–300 K for radicals in natural calcite samples obtained from a cave stalactite and a dripstone layer. Four types of carbonate radical spectra and two sulfate radical spectra were identified and high accuracy g-factors were derived. Time and temperature behaviour of the spectra show that the dominating CO2− radicals are rigidly bonded or undergo free reorientations, whereas CO3−, SO2− and SO3− only undergo free reorientations. Below 200 K the free reorientations of CO2− are suppressed and a hindered rotation around single local axis appears. The ESE detected spectrum proves that the lines of free rotating radicals are homogeneously broadened, thus they cannot participate in electron spin echo formation. Spin–lattice relaxation data show that CO2− radicals are decoupled from lattice phonons and relax via local mode tunnelling motion between inequivalent oxygen positions of CO2− molecules. The tunnelling appears in two excited vibrational states of energy 71 and 138 cm−1. Librational motions of CO2− molecules were detected by electron spin echo decay (phase relaxation) with energy 153 cm−1. Two kinds of impurity hydrogen atoms were distinguished from ESEEM: in-water inclusions and water coordinated to the calcium ions.