Detection of narrow lines in the inhomogeneously broadened line of P1 centers in diamond by double modulation EPR spectroscopy

The study of double modulation continuous wave (CW) electron spin resonance (DM-EPR) is reported to detect narrow “spin-packet”-like lines within an inhomogeneous CW-EPR line of low concentration P1 (N_s⁰) centers ([N_s⁰] < 1 ppm) in diamond. These narrow lines appear as peaks at multiples of modulation frequency in the double modulation spectrum. It is shown that the respective line shape can be fitted assuming two Lorentzian line shapes. The linewidth of the narrower Lorentzian is comparable with the linewidth of the Lorentzian obtained from the $T2$ relaxation time $δ1=1/πT2$ estimated from the Hahn echo pulsed-EPR experiment. The broader component $δ2$ correlates with the $T2*$ relaxation time obtained from the free induction decay measurement of the inhomogeneous spectral line shape of the P1 center. The ratio $δ2/δ1∼18$ derived from DM-EPR measurements approaches within the experimental error the value $T2/T2*∼16$ derived from pulsed-EPR. Sensitivity to the small shift of the magnetic field (⁠$∼0.6$ μT) was shown to be detectable from the broadening behavior of the $δ2$ component. The results promote DM-EPR as a method complementary to pulsed-EPR for studying coherence times of low density paramagnetic centers in a diamond lattice like nitrogen-vacancy (NV⁻) center, since it exhibits similar properties to the here investigated P1 center in terms of long relaxation time at a monitored low concentration.