The ESR of oxalic acid dihydrate irradiated by X-ray and by high-intensity pulsed ultraviolet light is studied at various frequencies ranging from 3.0 GHz to 36.9 GHz. The changes in a spectrum, a singlet at ∼36.9 GHz, a doublet at ∼3.0 GHz, and a triplet at ∼9.6 GHz, are associated with weak anisotropic interactions between electronic and nuclear spins. The latter give a satellite or hyperfine structure depending on the value of H0. The values of intensities and splittings agree with estimates made by considering quantization of the proton in effective magnetic fields.
