Low-Frequency NARS (LF NARS) by the use of a superheterodyne EPR spectrometer

Abstract

In this article, the possibilities of recording EPR spectra by the non-adiabatic rapid sweep (NARS) method on a superheterodyne spectrometer are investigated. This method allows recording the pure EPR absorption spectrum of the object under investigation without the need for lineshape–lineheight compromise, while increasing sensitivity by suppressing low-frequency noise. In the NARS method, a low-amplitude sinusoidal modulation of a magnetic field is not used, and an oscillating triangular-shaped bipolar magnetic field is superimposed on the main permanent magnetic field. The triangular field repetition rate should be higher than that of the undesirable noise, and the amplitude is such that the rate of field change satisfies the Bloch non-adiabaticity criterion. The EPR absorption signal is digitized by a fast ADC and accumulated over a large number (n) of triangular field periods. In this case, for low-frequency noise, the spectrum of which is located below the repetition frequency of the triangular field, an accumulation process occurs with an increase in the signal-to-noise ratio (SNR) in proportion to n. A remarkable property of a superheterodyne spectrometer is that the frequency below which low-frequency noise prevails over white noise is significantly lower for it than for a homodyne spectrometer. This allows the use of a low repetition rate (LF NARS), which makes it much easier to obtain a highly linear triangular field of significant amplitude even in microwave resonators with a massive metal case since the harmful effects of eddy currents are reduced. The conditions of non-adiabaticity become easily feasible. At the same time, the noise suppression effect during accumulation turns out to be so significant that the SNR of the LFNARS spectrum exceeds that of the traditional spectrum with magnetic field modulation, with the same recording time, by more than 10 times.