Spinrelaxationer

Spinrelaxationer are processes by which the spin degrees of freedom of a quantum system return to thermal equilibrium after an external perturbation, such as a pulse of radiofrequency radiation used in magnetic resonance experiments. In most contexts, the term refers to spin relaxation phenomena in nuclear, electronic, or atomic spins and is analyzed in terms of two characteristic relaxation times: T1 (spin-lattice, longitudinal) and T2 (spin-spin, transverse).

T1 describes the time scale over which the net magnetization along the external magnetic field recovers its equilibrium value by exchanging energy with the lattice; T2 describes the decay of transverse magnetization due to dephasing and loss of phase coherence among spins, without necessarily exchanging energy with the lattice.

Relaxation arises from interactions that perturb spin energy levels: magnetic dipole-dipole coupling between spins; coupling to lattice vibrations (phonons); chemical shift anisotropy; exchange interactions; and for electron spins, spin-orbit coupling. The relative importance depends on the system, temperature, and magnetic field.

Experimentally, T1 is commonly measured using inversion-recovery or saturation-recovery sequences, while T2 is obtained from spin-echo or CPMG sequences. The Bloch equations provide a phenomenological description of these processes, linking relaxation times to the decay of magnetization components toward equilibrium.

Applications include medical imaging (MRI relies on T1 and T2 contrast), chemical and material characterization by NMR and ESR, and studies in quantum information where spin coherence times set performance limits.

Note on terminology: "spinrelaxationer" is not standard English; most literature uses "spin relaxation processes" or simply "spin relaxation."