 If the spin populations are perturbed by an appropriate RF field at the transition energy frequency, the spin populations return to thermal equilibrium by a process called "spin-lattice relaxation".The rate of transitions from α to β is proportional to the population of state α, "P"α, and is a first order process with rate constant "W".The condition where the spin populations are equalized by continuous RF irradiation ("P"α = "P"β) is called "saturation" and the resonance disappears since transition probabilities depend on the population difference between the energy levels.