Spin-Lattice Relaxation Phenomena

Abstract

More than fifty years ago, Einstein showed that induced transitions between a given pair of energy states occur with equal probability in each direction. At microwave frequencies one can consider merely the effect of a radiation field stimulating transitions in each direction with equal probability, and neglect the effect of spontaneous emission, which may be several orders of magnitude less probable. One observes resonance absorption (for example in electron spin resonance) because the populations of the two levels are not equal. A Boltzmann distribution occurs with the population of the lower level greater than that of the upper level. Since the number of transitions which take place in unit time from a certain level is just equal to the population of that level multiplied by the probability for each transition, there is a nett absorption of energy from the radiation field. If this were the only process taking place, clearly the populations would rapidly be equalized, and resonant absorption would cease. Since the system must return eventually to the original Boltzmann distribution when the cause of the disturbance is removed, there must be a mechanism acting to restore equilibrium; such a mechanism is called a ‘relaxation process’.