How does spin-lattice relaxation return hydrogen nuclei to their lower-energy states?

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Spin-lattice relaxation is the process by which a system of hydrogen nuclei returns to its equilibrium distribution after being perturbed by electromagnetic radiation.1

When hydrogen nuclei are placed in an external magnetic field, they can occupy either a lower-energy (parallel) or a higher-energy (antiparallel) spin state. If the equilibrium distribution is disrupted, the nuclei return to their lower-energy state through time-dependent, radiationless transitions.1

During this process, the excess energy held by the nuclei in the upper spin state is transferred to the surrounding molecules. This energy is dissipated as increased translational, rotational, or vibrational energy within the surrounding environment, often referred to as the "lattice". Unlike spin-spin relaxation, which involves an exchange of spins between individual nuclei, spin-lattice relaxation is responsible for restoring the overall equilibrium distribution of the population.1