Irreversibility in Interacting Spin Systems

A system whose Hamiltonian is split into two terms $\mathcal{H}={\mathcal{H}}_0+\lambda V$ exhibits two types of irreversible processes. The first processes are described by ${\mathcal{H}}_0$ alone; only the second processes, which result from the pertubation, lead to an increase of the entropy of the system. These processes are illustrated by the examples of free precession and cross-relaxation. General formulas are given for transition probabilities and the expressions, applied to cross-relaxation in LiF, agree with the results obtained by Bloembergen and Pershan.