ABSTRACT
This chapter provides elementary tools for the description of spin systems and their equilibrium and non-equilibrium states. The concepts of equilibrium and deviation from equilibrium have a central role in physical chemistry. Using experimental techniques of nuclear magnetic resonance (NMR) spectroscopy, in the first place radiofrequency pulses, it is possible to create a large variety of non-equilibrium states. In NMR, the processes by which the return to equilibrium is achieved are denoted spin relaxation. The concepts of relaxation and equilibrium are closely connected to the behaviour of macroscopic samples of spins. Relaxation experiments were among the earliest NMR applications of modern Fourier transform NMR, and the development of applications and experimental procedures has been enormous. Relaxation is important; it has effects on how NMR experiments are carried out, but perhaps even more valuable is the information content derived from relaxation parameters.
