Electron spin resonance of an X-ray irradiated single crystal of potassium dihydrogen arsenate, KH2AsO4

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Abstract
The electron spin resonance of an x-ray irradiated single crystal of potassium dihydrogen arsenate has been examined and a radical which is thought to be AsO4 4- identified. Analysis of the spectra for various orientations of the crystal with respect to the external magnetic field leads to an axially symmetric hyperfine interaction tensor which is in agreement with what would be expected for the AsO4 4- radical. Semi-empirical LCAO molecular orbital calculations for the AsO4 4- radical with symmetry D 2d trapped in the tetragonal KH2AsO4 crystals leads to the conclusion that the odd electron enters a molecular orbital with A 1 symmetry. The isotropic part of the hyperfine interaction tensor is calculated from this theory to be 2098 Mc/s which must be compared with the experimental value of 3038·6 Mc/s, The anisotropic part of the hyperfine interaction calculated from the theory, however, is much too low when compared with the value estimated from the experimental data. It is possible that part of this difference is due to the neglect of spin polarization effects. Proton hyperfine structure has been resolved on each of the lines due to the 75As nucleus. For the case with the c axis of the crystal orientated parallel to the magnetic field five lines are observed in the intensity pattern of 1 : 4 : 6 : 4 : 1. This supports our viewpoint that the radical is AsO4 4- hydrogen-bonded to other tetrahedra units in the crystal.