Continuous wave multi-quantum electron paramagnetic resonance spectroscopy

The equation of Anderson [Phys. Rev. 102, 151 (1956)] (ω2−ω1)2=(γH0−ω1)2+γ2H21, which describes resonance conditions if relaxation times are long and irradiation at two frequencies is applied to a spin system, has been studied experimentally in the context of continuous wave electron paramagnetic resonance (EPR) spectroscopy. Here ω2 and ω1 are the frequencies of two incident microwave levels, one of which is much stronger than the other and is of amplitude H1. γH0 is the resonant condition if just one frequency is applied. Magnetization at either ω1 or ω2 has been observed as a function of sweep of the static magnetic field, sweep of ω2 and also sweep of the amplitude H1. Observation of magnetization at frequency ω1 corresponding to the strong microwave field H1 replicates the rotary saturation experiment of Redfield [Phys. Rev. 98, 1787 (1955)]. Multi-quantum effects are studied with the two frequencies well separated and also when they lie within the width of a single homogeneous line. In addition, data are shown when both microwave amplitudes are similar and the Anderson equation is no longer correct. The thrust of the work is not only to study the spin physics, but also to develop a basis for our development of rotary resonance as an alternative to field modulation in EPR spectroscopy [J. Chem. Soc. Faraday Trans. 1 85, 3901 (1989)].