  We report on an analytical description of spin-dependent electronic
transition rates which are controlled by a radiation induced spin-Rabi
oscillation of weakly spin-exchange and spin-dipolar coupled paramagnetic
states (S=1/2). The oscillation components (the Fourier content) of the net
transition rates within spin-pair ensembles are derived for randomly
distributed spin resonances with account of a possible correlation between the
two distributions that correspond to the two individual pair partners. The
results presented here show that when electrically or optically detected Rabi
spectroscopy is conducted under an increasing driving field B_ 1, the Rabi
spectrum evolves from a single resonance peak at s=\Omega_R, where
\Omega_R=\gamma B_1 is the Rabi frequency (\gamma is the gyromagnetic ratio),
to three peaks at s= \Omega_R, s=2\Omega_R, and at low s<< \Omega_R. The
crossover between the two regimes takes place when \Omega_R exceeds the
expectation value \delta_0 of the difference of the Zeeman energies within the
pairs, which corresponds to the broadening of the magnetic resonance lines in
the presence of disorder caused by hyperfine field or distributions of Lande
g-factors. We capture this crossover by analytically calculating the shapes of
all three peaks at arbitrary relation between \Omega_R and \delta_0. When the
peaks are well-developed their widths are \Delta s ~ \delta_0^2/\Omega_R.
