Phosphorescence
Definition and meaning of Phosphorescence in chemistry.
Phosphorescence is the emission of light from a substance that continues after the excitation source has been removed, typically lasting from milliseconds to several hours. It occurs when a substance absorbs photons and transitions through a long-lived, metastable excited state before emitting photons.
In more detail
Phosphorescence differs fundamentally from fluorescence because it involves a forbidden electronic transition. When a molecule absorbs a photon, it typically reaches an excited singlet state, which can undergo an intersystem crossing to a lower-energy triplet state. The return from the triplet state to the ground singlet state is quantum mechanically forbidden, making it a slow process. This slow decay allows the characteristic afterglow to persist long after the exciting light source is removed.
Key facts
| Field | Physical Chemistry |
|---|---|
| Emission duration | Milliseconds to hours, depending on the material |
| Key mechanism | Forbidden transition from metastable triplet state to ground singlet state |
| Common phosphor compounds | Zinc sulfide (ZnS) with copper or silver activators (shorter afterglow); strontium aluminate (SrAl2O4) with europium/dysprosium (longer afterglow) |
Zinc sulfide (ZnS) doped with copper or silver activators is a classic phosphorescent pigment used in inexpensive glow-in-the-dark paints, ceramics, and decorative items; after exposure to sunlight or ultraviolet radiation it typically glows for only about 30 to 60 minutes before fading. Modern glow-in-the-dark products more often use strontium aluminate (SrAl2O4) doped with europium and dysprosium, which glows several times brighter and can remain visible for 8 to 12 hours.
Frequently asked questions
How does phosphorescence differ from fluorescence?
Fluorescence is fast (typically on the order of nanoseconds) and involves an allowed transition from an excited singlet state directly to the ground state. Phosphorescence is slow because it involves a forbidden transition from a metastable triplet state, allowing the glow to persist for milliseconds to hours after the excitation source is removed.
Why do glow-in-the-dark materials glow?
These materials contain phosphorescent compounds, such as copper- or silver-activated zinc sulfide or europium-doped strontium aluminate, that absorb visible or ultraviolet light and store the energy in long-lived triplet states. This energy is gradually released as visible light through forbidden transitions, with the visible afterglow lasting anywhere from under an hour for zinc sulfide-based pigments to more than eight hours for strontium aluminate-based ones.