Relaxation Mechanisms:

The population of the energy levels is governed by Boltzmann distribution law. At one a radiation is absorbed then the population distribution gets disturbed. Since the population difference among the two spin states is extremely small, the population of lower and higher energy levels would become equal after absorption of radiation for some time. It means which resonance signal has achieved saturation and no more energy could be absorbed.  However, there are some competing phenomena that do not allow this stage to come. These are called relaxation mechanisms. In the process of relaxation the nuclear spins in the excited state relax down to the lower state through radiationless processes. There are of two types of relaxations namely, spin-lattice relaxation or spin-spin relaxation. In sequence to manage the population difference within the energy levels, the rate of relaxation from the excited state to the lower energy level must be greater than the rate at which radiofrequency is absorbed. Now let us learn about these relaxation mechanisms.