multiplet

A multiplet is a term used in spectroscopy and related fields to describe a group of closely spaced spectral lines or energy states that are treated as a single set because they originate from a common transition or configuration but are split by interactions such as spin coupling or fine structure. The concept emphasizes that what appears as multiple lines reflects an underlying simple origin.

In optical and atomic spectroscopy, a multiplet consists of several lines arising from a single electronic transition that are split by spin-orbit coupling, spin-spin interactions, or hyperfine effects. The lines typically share a common upper and lower energy configuration but differ in angular momentum coupling, resulting in a characteristic pattern that can reveal information about the electronic structure and the environment of the atom or molecule.

In nuclear magnetic resonance and other magnetic resonance techniques, a multiplet refers to the splitting of a resonance into multiple lines due to coupling with nearby magnetic nuclei. A common example is a signal split into a quartet by coupling to three equivalent protons. More generally, the multiplicity depends on the number and type of coupled spins and can be more complex when non-equivalent nuclei are involved. The intensities of the lines often follow predictable patterns, such as those described by Pascal’s triangle in simple cases.

In nuclear physics, the term also denotes a set of related nuclear states with the same total quantum number (such as isospin) but differing in another quantum number. These isospin multiplets reflect underlying symmetries of the nuclear system.

Overall, multiplets provide a framework for interpreting spectral structures and energy-level schemes by grouping related components that arise from a common origin but are split by interactions.