Illustrated Glossary of Organic Chemistry

Relaxation: The process of changing from an excited state to a lower energy state (often the ground state) by transferring energy to other atoms, and/or photon emission.

In NMR spectroscopy, absorption of a radio wave photon of energy ΔE causes excitation from the ground nuclear spin state (I = +1/2) to the excited nuclear spin state (I = 1-/2). The excited nucleus then relaxes by transferring energy to other atoms around it. Observing this nuclear relaxation is the basis for the 2D-NMR technique called NOESY, and for MRI (magnetic resonance imaging).