ABSTRACT
In the introductory chapters it has been shown that for I = 1/2 nuclei, there are two discrete energy levels to be taken up by spins, with the majority of the spins populating the lower energy level (in excess to the upper energy level population by approximately 1 ppm). After a saturation recovery or an inversion recovery experiment, the Boltzmann distribution is perturbed. Return of the spin population back to equilibrium occurs via transition of nuclei from the higher energy level back to the low energy level via either a spontaneous (optical spectroscopy) or a stimulated emission. The probability for a spontaneous emission in the radio frequency (RF) range of frequencies is nonsignificant. Such transitions, accompanied by a loss of quanta of energy ΔE, restore the Boltzmann distribution, and the process is known as spin–lattice relaxation.
