Fluorescence
Definition and meaning of Fluorescence in chemistry.
Fluorescence is the rapid emission of light by an atom or molecule that has absorbed electromagnetic radiation, occurring within nanoseconds as an excited electron falls back to the ground state.
In more detail
Absorption of a photon promotes an electron from the ground state (S0) to an excited singlet state (S1). The molecule quickly loses some energy as heat through vibrational relaxation, then returns to S0 by emitting a photon of lower energy, and therefore longer wavelength, than the one absorbed; this wavelength difference is called the Stokes shift. Because the S1-to-S0 transition is spin-allowed, emission is fast (roughly 1-100 nanoseconds), which distinguishes fluorescence from phosphorescence, where a spin-forbidden triplet-to-singlet transition delays emission far longer.
Key facts
| Field | Physical Chemistry |
|---|---|
| Typical lifetime | ~1-100 nanoseconds |
| Transition type | Spin-allowed (S1 to S0) |
| Key feature | Stokes shift: emitted light has longer wavelength than absorbed light |
A quinine sulfate solution, colorless in ordinary light, glows bright blue under ultraviolet light because it absorbs UV photons and re-emits the energy as visible fluorescent light.
Frequently asked questions
How does fluorescence differ from phosphorescence?
Fluorescence is a spin-allowed transition from an excited singlet state that decays within nanoseconds, while phosphorescence involves a spin-forbidden transition from a triplet excited state and can persist for milliseconds to hours, producing an afterglow after the light source is removed.
Why is fluorescent light usually a different color than the light absorbed?
Some absorbed energy is lost as heat through vibrational relaxation before the photon is emitted, so the emitted photon has lower energy and longer wavelength than the absorbed one; this is the Stokes shift.