The Bruker lecture alternatives to field modulation in electron spin resonance spectroscopy

Abstract

In conventional ESR spectrometers, high-frequency magnetic field modulation is transferred to microwave amplitude and frequency modulation by the spin system, thereby overcoming low-frequency noise arising from the oscillator, the detection system and the environment. A serious disadvantage of field modulation arises from the compromise forced on the spectroscopist by lineshape and signal-amplitude considerations. Over the years we and others have investigated alternatives to field modulation, but success has been minimal. The invention of the loop-gap resonator (LGR) with its very high energy density per watt of incident power and its low Q gives rise to technological opportunities that have led us to reinvestigate several of these alternatives, and these studies are reported here. It is true that the properties of the LGR give rise to improved performance in some of these alternative strategies, compared with cavity resonators. More significantly a new approach for avoidance of field modulation in ESR spectroscopy has been discovered which is called Electron Paramagnetic Rotary Resonance. The method is based on resonance that occurs when the difference of two incident microwave frequencies matches the precession frequency in the rotating frame, following an analysis of Bloch. Pure absorption or dispersion lines (i.e. not derivatives) are observed with good baseline stability and freedom from microwave source noise. It is a transverse magnetization detection scheme, and not longitudinal. The hypothesis is made, based on first experimental results, that the method is a general one that can provide a viable alternative to field modulation.