NONLINEAR ESR SPECTROSCOPIES IN ORDERED FLUIDS

The non-linear ESR response of a paramagnetic system to multiple irrad iation provides a powerful tool to investigate the longitudinal spin r elaxation. The extended observation time increases the sensitivity to random processes with long correlation time which affect the spin rela xation. With the aid of a simple model of a spin probe in an ordered f luid, we outline the main opportunities offered by non-linear ESR. Exa ct analytical expressions are found which exhibit a marked dependence of the linewidth to the details of the dynamics of the probe.