The spectrum of continuous-wave electron paramagnetic resonance (CW-EPR) was analyzed with respect to the shape of the line under microwave (MW) power saturation ( \({P}_{MW}\sim {B}_{1}^{1/2};{B}_{1}\) is intensity of MW magnetic field). The used \({B}_{1}\) increases until the Rabi frequency \(({\omega }_{1}=\gamma {B}_{1}\) , \(\gamma\) is the electron gyromagnetic ratio) approaches the radiofrequency \(\left({\omega }_{rf}\right)\) used as the magnetic modulation frequency. The possible effect of Rabi resonance \(\left({\omega }_{1}={\omega }_{rf}\right)\) , on spin packets system can be detected under the condition of “weak modulation near the Rabi resonance” ( \({\omega }_{2}\ll {\omega }_{1}\approx {\omega }_{rf})\) where \({\omega }_{2}\) is modulation amplitude of radiofrequency field expressed in frequency units. Inhomogeneously broadened, CW-EPR, lines of the P1 (Ns0) nitrogen centers (30 ppm > [Ns0] < 200 ppm) in diamond crystal and E’ defect in irradiated vitreous SiO2 with long spin relaxation times ( \({T}_{1}\) and \({T}_{2}\) ranged in the interval from around 2 ms to 1 µs at room temperatures) were selected for saturation study. Spectra were recorded using magnetic field modulation \(({\omega }_{rf}/2\pi =100\) kHz) with modulation amplitude \(({\omega }_{2}/2\pi =27\text{ kHz})\) as the first harmonic for two phase-sensitive detections, detection in-phase with respect to the modulation frequency and detection 90° out-of-phase with respect of the modulation frequency. For the later detection, the resulting change near Rabi resonance can be related to the expected inversion of modulation sideband lines under resonance conditions. This effect detected in the saturation process on inhomogeneous spectral lines of P1 and E’ centers shows that both samples can be used as a standard for estimation \({B}_{1}\) value.
