Selection Rule
Definition
A quantum-mechanical rule that decides whether a transition between two levels is allowed or forbidden. Allowed transitions are intense and fast; forbidden transitions are weak and slow. Phosphorescence is slow because the singlet-to-triplet transition behind it is spin-forbidden.
- Domain
- Quantum mechanics / spectroscopy
- Decides
- Whether a transition is allowed or forbidden
- Effect
- Allowed transitions are fast and intense, forbidden ones weak and slow
- Example
- Spin-forbidden singlet-to-triplet transition in phosphorescence
Common questions
Why does a forbidden transition still happen at all if it is forbidden?+
Selection rules describe strongly favoured transitions under ideal conditions, but weaker coupling mechanisms can still allow a forbidden transition to occur, just at a much lower probability and over a longer timescale.
How does this explain the delay between fluorescence and phosphorescence?+
Fluorescence involves an allowed, spin-conserving transition that happens almost instantly, while phosphorescence involves a spin-forbidden transition that is slow, which is why phosphorescent afterglow can persist after the light source is removed.
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