singlet

A singlet is a quantum state with total spin angular momentum S = 0, giving a spin multiplicity of 2S + 1 = 1. In a system of two spin-1/2 particles, the singlet state is the antisymmetric combination (|↑↓⟩ − |↓↑⟩)/√2, in contrast to the triplet states (|↑↑⟩, (|↑↓⟩ + |↓↑⟩)/√2, |↓↓⟩) which have S = 1.

In chemistry, a singlet electronic state refers to a configuration in which all electrons are paired and the total spin is zero. Most closed-shell molecules in their electronic ground state are singlets. Excited electronic states can be singlet or triplet; transitions between singlet states are typically spin-allowed, while singlet-to-triplet transitions involve a spin flip and are often weaker.

In spectroscopy and photophysics, singlet and triplet states underlie fluorescence and phosphorescence. Emission from a singlet excited state is fluorescence, usually fast, whereas emission from a triplet state is phosphorescence, typically slower due to spin-forbidden transitions. Inter-system crossing, aided by spin-orbit coupling, enables population transfer between singlet and triplet manifolds.

Singlet oxygen refers to the electronically excited state of molecular oxygen (O2) with total spin zero. It is a reactive species generated by photosensitizers and has distinct chemical reactivity from ground-state triplet oxygen.

In particle physics and related fields, the term singlet denotes a state that does not transform (or transforms trivially) under a given symmetry group, such as a color-singlet state in quantum chromodynamics or a scalar singlet under a symmetry.