spinspinrelaxation

Spin-spin relaxation, also referred to as transverse relaxation, is the process by which the transverse component of magnetization decays after excitation. It arises from the loss of phase coherence among spins in a sample and is characterized by the time constant T2. In an imperfect or inhomogeneous magnetic field, the observed decay can include an additional contribution from field inhomogeneity, often described by T2*, which is shorter than the true T2.

The principal microscopic mechanism behind spin-spin relaxation is dipole-dipole coupling between nearby spins, modulated by molecular motion. This interaction causes the precession frequencies of individual spins to wander relative to one another, leading to dephasing. Other contributing factors include chemical shift anisotropy, scalar J-coupling pathways that feed relaxation, and chemical exchange processes. In liquids, rapid molecular tumbling averages many interactions and generally lengthens T2, while in solids these interactions are not averaged out and T2 tends to be much shorter.

T2 is distinct from T1, the spin-lattice relaxation time that governs recovery of longitudinal magnetization. In general, T2 ≤ T1. T2 is measured using pulse sequences that refocus dephasing, such as the Hahn echo or Carr-Purcell-Meiboom-Gill (CPMG) sequences, which suppress inhomogeneous broadening to reveal the true transverse relaxation. The reciprocal, R2 = 1/T2, is the transverse relaxation rate.

Spin-spin relaxation is a central concept in NMR and ESR, influencing spectral linewidths, relaxation-based experiments, and contrast in magnetic resonance imaging and spectroscopy.