spinforbidden

Spinforbidden refers to electronic transitions that are forbidden by the spin selection rule for electric dipole transitions, meaning the total spin quantum number S changes during the transition (ΔS ≠ 0). In practice, this occurs between states of different spin multiplicities, such as singlet to triplet transitions, which have very small oscillator strengths for electric dipole (E1) processes.

Because spin is not conserved in these transitions under the E1 operator, spinforbidden lines are typically weak in absorption or emission spectra and are often associated with long-lived excited states. The observed intensity of such transitions increases with spin–orbit coupling, which mixes spin states and partially relaxes the selection rule, a phenomenon that becomes more pronounced in heavier elements.

In molecular systems, spinforbidden transitions underlie processes like intersystem crossing, where population moves from a singlet excited state to a triplet manifold. This pathway can lead to phosphorescence or delayed fluorescence, depending on the subsequent radiative or non-radiative decay channels. Beyond E1, spin-forbidden transitions can occur via higher-order processes such as magnetic dipole (M1) or electric quadrupole (E2) transitions, or through vibronic coupling that mixes electronic and vibrational states.

Overall, spinforbidden transitions illustrate how spin and relativistic effects influence spectroscopy, lifetimes, and radiative pathways. Related concepts include spin selection rules, intersystem crossing, and phosphorescence.