Sub-collision hyperfine structure of nonlinear-optical resonance with   field scanning
  Some experimental evidences for methane are produced that the simple transition from frequency scanning of nonlinear-optical resonances to magnetic one may be accompanied with transition from sub-Doppler collisionally broadened structure to sub-collision hyperfine one. It is conditioned by nonlinearity of splitting of hyperfine sublevel for molecules in the adiabatically varied magnetic field and respectively breaking the analogy of magnetic and frequency scannings. The exact calculation of the resonance structure is considered for molecules with only one spin subsystem. The approximately spin-additive calculation of the structure is given for sufficiently fast rotating molecules with greater number of spin subsystems. Within the same approximation an example of hyperfine doubling in the magnetic and electric spectra of nonlinear-optical resonance is considered for fluoromethane. 
K. I. Gus'kov, V. I. Kovalevsky, A. G. Rudavets, E. G. Saprykin