Modulation sidebands spectra within inhomogeneous CW-EPR line detected by double modulation EPR spectroscopy

Conventional modulation spectrum, MS, in continuous wave electron paramagnetic resonance, CW-EPR, is produced by applying longitudinal radiofrequency (RF) fields with the frequencies, ω_rf, which exceeds the linewidth, 2πδ, of a single spectral line given in frequency units. The second longitudinal RF field with frequency, ω_rf2, is employed to produce double modulation spectrum, DMS. In this work DMS are presented as a specific type of MS which can be produced from an ordinary homogenous line. The numerical simulations of DMS in the limit of low power saturation relied on the recently introduced multi-photon transitions formalism which includes one microwave photon in combination with an arbitrary number of radiofrequency photons. It is shown that DMS of an inhomogeneous line exhibits similar basic structure as MS and exhibits sideband peaks at multiples of basic radiofrequencies. Linewidths of these peaks are significantly narrower (cca. two - three orders of magnitude) than the inhomogeneous linewidth and can be correlated with the underlying homogeneous linewidth components on the basis of characteristic spin-lattice, T₁, and spin-spin, T₂, relaxation times. The capability to extract T₁ and T₂ from DMS was tested on the well-known E' defect in irradiated vitreous SiO₂. The obtained results revealed the impact of "rapid passage" effect on DMS in improving the detection sensitivity of DMS in the study of paramagnetic centers with long relaxation times. Therefore, double modulation method can be considered as a complementary method for studying inhomegeneous broadening in the EPR spectra.