Continuous wave multiquantum electron paramagnetic resonance spectroscopy. II, Spin-system generated intermodulation sidebands

Irradiation of a single homogeneous electron paramagnetic resonance (EPR) transition by two microwave sources separated by Δf, where Δf≪ the linewidth, results in the production of intermodulation sidebands at f0±(k+1/2)Δf, where f0 is the mean of the two irradiating frequencies and k is an integer ≥1. These sidebands can be seen directly with a microwave spectrum analyzer. Any one of the sidebands can be observed by suitable phase sensitive detection and displayed as a function of the polarizing magnetic field H0, resulting in a multiquantum EPR spectrum. The widths of the EPR lines decrease as k increases, resulting in improved resolution. The two k=1 transitions can be superimposed, resulting in a signal of double intensity. At saturating microwave power, this double-intensity line is two times less intense than an ordinary EPR line. No magnetic field modulation needs to be used when detecting multiquantum transitions, and pure absorption or pure dispersion signals are obtained with good baseline stability. Considerable attention is paid to the technical problem of irradiating the sample with just two microwave frequencies. In the apparatus described, spurious instrumental sidebands are reduced by 60 dB or more.