ABSTRACT
The description of relaxation has been focused on simple T1 and T2 relaxation processes and the somewhat more complicated phenomena of cross-relaxation and the nuclear Overhauser effect. A more general formulation of relaxation theory, suitable also for systems with scalar spin-spin couplings, is known as the Wangsness, Bloch and Redfield theory or the Redfield theory. The Redfield formulation of relaxation theory has the advantage of being applicable to any type of non-equilibrium system and any relaxation mechanism. The Redfield relaxation theory is often denoted as semiclassical theory. The non-secular terms are less efficient in causing relaxation, because they oscillate rapidly, which effectively averages them to (almost) zero. The relaxation-generating perturbation is the dipole-dipole interaction. The relaxation properties in a coupled two-spin system are also sensitive to the presence of anisotropic chemical shielding, another interaction leading to relaxation. The Liouville space formulation of relaxation theory makes use of the Liouvillian, a superoperator counterpart of the Hamiltonian.
