Analytical study of spin - dependent transition rates within pairs of dipolar and strongly exchange coupled spins with s = 12 during magnetic resonant excitation

We study theoretically the spectrum F(s) of spin-dependent transition rates within dipolar D and exchange J coupled pairs of two spins with S = 12 undergoing Rabi oscillations due to a coherent magnetic-resonant excitation. We show that the Rabi oscillation controlled rates exhibit a spectrum with three frequency components. When exchange J is stronger than the driving field R (in the frequency units), the frequency components of the Rabi oscillation do not depend on J , rather they are determined by the relation between the R and D. We derive analytical expressions for the frequencies and the intensities of all three Rabi oscillation components as functions of R/D and δ/D, where δ is detuning of the driving ac field from the Larmor frequency. When R D, the two lower frequencies approach s = R , while the upper line approaches s = 2 R . Disorder of the local Larmor frequencies leads to a Gaussian broadening of the spectral lines. We calculate corresponding widths for different R/D and δ/D. Unexpectedly, we find that one of the frequency components exhibits an unusual evolution with R: its frequency decreases with R at R < D. Upon further increase of R , this frequency then passes through a minimum and, eventually, approaches s = R . Nonmonotonic behavior of the frequencies is accompanied by nonmonotonic behavior of the respective oscillation intensity.