RELATIONSHIP BETWEEN A NONLINEAR RESPONSE AND RELAXATION INDUCED BY COLORED NOISE

The nonlinear response to external disturbances of a system of interes t A (the ''atom'') in contact with a thermal bath is studied. The cent ral issue is the relationship between the nonlinear susceptibility and the equilibrium relaxation spectra of observables of A. Paralleling t he results of the linear-response theory, the nth-order nonlinear susc eptibility is factorized by quantum-mechanical arguments into two term s pertaining to the disturbances and the spin-bath system, respectivel y. The role of slow atom states, i.e., states relaxing on times T long er than the bath correlation time tau(c), is pointed out, by recurring to recent findings on their general features [Phys. Rev, A 46, 6222 ( 1992)]. In the frame of a stochastic picture of the bath and a multile vel scheme of the system A, the conditions under which the nonlinear s usceptibility provides the spectrum of the slow atom states are clarif ied. Illustrations are drawn from magnetic resonance to demonstrate th at techniques based on the nonlinear response of a spin system to mult iple continuous waves compete favorably with pulsed techniques to prov ide information on longitudinal spin relaxation.