Cross-relaxation and constants of the motion

The dependence of the longitudinal dynamic susceptibility on the static magnetic field is studied theoretically. The susceptibility is expressed in terms of the collision operator. Eigenvectors of the collision operator with respect to zero eigenvalue are investigated by using perturbation theory. It is shown that the real part of the susceptibility coincides with the adiabatic susceptibility at harmonic fields and its dependence on the magnetic field near harmonic fields is of the Lorentz type.