g-Sign determination with circularly polarised RF fields

We have used a pair of crossed coils to generate rotating RF fields for a muon spin resonance measurement by driving one of the two coils and adjusting the tuned frequencies and mutual coupling to give the required 90-bar phase shift. Fitting the measured impedance to an equivalent circuit model gives information on the rotating field. Test experiments on muonium in quartz confirmed the ratio of the two values of rotating field components B1. We have then used the probe to study the shallow donor states in some II-VI semiconductors by double resonance with the electrons, where the g-factor of the electron differs from +2 and in some cases can be negative. Experiments so far have used longitudinal fields, where the double resonance can only be observed up to about 20G or 50MHz