Editing Resonance (section)

=== Atomic, particle, and molecular ===
{{Main|Nuclear magnetic resonance|Resonance (particle physics)}}

[[File:HWB-NMR - 900MHz - 21.2 Tesla.jpg|thumb|upright|[[Nuclear magnetic resonance|NMR]] Magnet at HWB-NMR, Birmingham, UK. In its strong 21.2-[[Tesla (unit)|tesla]] field, the proton resonance is at 900&nbsp;MHz.]]
[[Nuclear magnetic resonance]] (NMR) is the name given to a physical resonance phenomenon involving the observation of specific [[quantum mechanics|quantum mechanical]] [[magnetism|magnetic]] properties of an [[atom]]ic [[atomic nucleus|nucleus]] in the presence of an applied, external magnetic field. Many scientific techniques exploit NMR phenomena to study [[molecular physics]], [[crystallography|crystal]]s, and non-crystalline materials through [[NMR spectroscopy]]. NMR is also routinely used in advanced medical imaging techniques, such as in [[magnetic resonance imaging]] (MRI).

All nuclei containing odd numbers of [[nucleon]]s have an [[Spin (physics)|intrinsic angular momentum]] and [[magnetic moment]]. A key feature of NMR is that the resonant frequency of a particular substance is directly proportional to the strength of the applied magnetic field. It is this feature that is exploited in imaging techniques; if a sample is placed in a non-uniform magnetic field then the resonant frequencies of the sample's nuclei depend on where in the field they are located. Therefore, the particle can be located quite precisely by its resonant frequency.

[[Electron paramagnetic resonance]], otherwise known as ''electron spin resonance'' (ESR), is a spectroscopic technique similar to NMR, but uses unpaired electrons instead. Materials for which this can be applied are much more limited since the material needs to both have an unpaired spin and be [[paramagnetic]].

The [[Mössbauer effect]] is the resonant and [[recoil]]-free emission and absorption of [[gamma ray]] photons by atoms bound in a solid form.

[[Resonance (particle physics)|Resonance in particle physics]] appears in similar circumstances to [[classical physics]] at the level of quantum mechanics and [[quantum field theory]]. Resonances can also be thought of as unstable particles, with the formula in the [[#Universal resonance curve|Universal resonance curve]] section of this article applying if ''Γ'' is the particle's [[Particle decay#Decay rate|decay rate]] and <math>\omega_0</math> is the particle's mass ''M''. In that case, the formula comes from the particle's [[Propagator (Quantum Theory)|propagator]], with its mass replaced by the complex number ''M''&nbsp;+&nbsp;''iΓ''. The formula is further related to the particle's decay rate by the [[optical theorem]].