The Overhauser-shift technique was used to investigate single crystals. By extending the high magnetic field of a W-band spectrometer, it was possible to resolve all spectral lines that were recorded and to assign them to their corresponding electronic and nuclear states. This separate analysis was the basis to access additional sample characteristics: the hyperfine coupling that was actually averaged out in the electron spin resonance signal, as well as the nuclear relaxation rates could be analyzed. Systematic measurements by varying the microwave power revealed the Overhauser shift in thermal equilibrium. The signal could be tracked to very small microwave saturation parameters, at which the deviation from the usual linear relation between power and shift became evident and the shift clearly approached a constant value. This value in equilibrium was determined directly by fitting a sequence of measurements. The probability densities of the electrons at the nuclei in the 2 non-equivalent crystallographic positions (lattice sites with octahedral and tetrahedral coordination) could also be determined directly. The enhanced resolution revealed a hidden sub-structure in the nuclear resonance signals. On the basis of a microscopic model, this structure could be used to probe the environment of the O vacancy more precisely and determine the extension of the electronic wave function of the donor electrons.

The Oxygen Vacancy in Ga2O3 - a Double Resonance Investigation. H.J.Kummerer, G.Denninger: Magnetic Resonance in Chemistry, 2005, 43, S145-52