Schonland ambiguity

Fig. 3.9 Schematic variation of the g-factor with the magnetic field in the xy-, yz-, and zx-planes in the Schonland method to determine the g-tensor. The variation is shown over a 90 = interval for the angle specifying the orientation of the magnetic field in each plane. For a perfect fit the measured g-factor should agree along the , , and axes in the different planes. A sign error for (g )xy occurs if the crystal is unintentionally mounted for rotation about -z in place of +z, see text for methods to resolve this Schonland ambiguity ...

An exact match may be difficult to achieve, experimentally, however. Some programs therefore have a feature for automatic adjustment of the positions of the axes. In crystals of symmetry higher than triclinic, spectra due to different sites that become identical along the crystal axes makes it easier to accurately determine the obtain axes as shown in Fig. 3.10. Another difficulty, related to the Schonland ambiguity then appears, however, as discussed next. 

This application may seem redundant, but is motivated e.g. by the so-called Schonland ambiguity in determining the hyperfine coupling tensor from single crystal measurements. It is only very recently that methods to eliminate the ambiguity [17] have been proposed, and the procedures are not always applicable. An investigation to clarify the radical structure of /-alanine is taken as an example in Fig. 3.27. 

All observed transitions for different ms and mi quantum numbers can be included simultaneously in the fit. This feature helps to avoid the ambiguity that may occur in the analysis with the Schonland procedure discussed above and in Section 3.2. Error estimates of the parameters are computed. The program also simulates transition frequencies for the rotation planes. The simulation results are automatically stored in files of two columns, field orientation (deg.) and transition frequency (MHz). Both rotation senses ( sites for orthorombic a,b,c and monoclinic crystals) are simulated. The program was developed by A. S0mes, and can be obtained at [http //www.fys.uio.no/biofysikk/eee/esr.htm]. 

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