spinallowed

Spinallowed is a term used in atomic and molecular spectroscopy to describe electronic transitions in which the total spin quantum number S is unchanged during the transition (ΔS = 0). Under the electric dipole selection rules, transitions that conserve spin are favored because they do not require a change in the spin state of the electrons. In atoms and molecules described by LS coupling, this means transitions within the same spin multiplicity, such as singlet to singlet or triplet to triplet states, are spinallowed. Transitions between states of different multiplicities, such as singlet to triplet, are spin-forbidden under strict E1 selection rules, and typically have much lower probability and longer lifetimes unless aided by coupling mechanisms.

Spin-orbit coupling can mix states of different spin, permitting weak transitions that would be spin-forbidden in pure LS coupling. This leads to phenomena such as intersystem crossing and phosphorescence, where emission can occur from a triplet state to the ground singlet, albeit weakly. In practice, spinallowed transitions generally produce stronger, shorter-lived emission (fluorescence) compared to spin-forbidden processes, though not all spinallowed transitions are highly intense; parity and angular momentum constraints can also limit their probability.

The concept is important for interpreting electronic spectra, photoluminescence quantum yields, and the design of materials for light-emitting devices. See also spin-forbidden, selection rules, spin-orbit coupling, intersystem crossing, fluorescence, and phosphorescence.