Electron-spin resonance

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Electrons, and other particles, have an intrinsic angular momentum, known as spin. This creates a magnetic dipole moment. When the electron is placed in a magnetic field, the intrinsic magentic dipole can align in one of two ways, parallel or anti-parallel to the field. The anti-parallel state is of lower energy. However, applying radiation of a certain frequency to the electron can raise it to the higher energy state, in which its magnetic dipole is parallel to the applied magnetic field. It will then fall back to the lower energy state, emitting a photon. If radiation continues to be applied, then the electron will "resonate" between the two energy states. This is known as electron spin resonance, and is used to identify compounds, which each have a unique spectrum of radiation absorption. This occurrence is used in both NMR and MRI