How does electromagnetic radiation disturb the equilibrium distribution of nuclear spin states?

In the context of nuclear magnetic resonance, certain atomic nuclei behave like tiny bar magnets when placed in an external magnetic field, aligning their magnetic moments either parallel or antiparallel to that field. The antiparallel orientation represents a higher-energy state, known as the upper spin state.12

The equilibrium distribution of nuclei between these parallel and antiparallel states is disturbed when the system absorbs electromagnetic radiation of an appropriate frequency. This absorption provides the necessary energy for the nuclei to "flip" from their lower-energy orientation into the higher-energy, upper spin state.21

Once this nonequilibrium distribution is established, the system attempts to return to its original equilibrium state through a process called spin-lattice relaxation. During this recovery, the excited nuclei dissipate their excess energy to the surrounding environment—transferring it as translational, rotational, or vibrational energy—to revert to the lower-energy orientation.2