What is Phosphorescence? Explain with example.
Many substances continue to emit light for quite some time even after the incident light stimulus is cut off. This slow, delayed emission of light is known as phosphorescence. Zinc Sulphide (ZnS) and the sulphides of alkaline earth metals (Group IIA) are excellent examples of such substances. Interestingly, the addition of a trace amount of a heavy metal enhances the intensities of light emission by a phosphorescent substance. Thus, the presence of specific impurities in the form of mixed crystals is often necessary for persistent phosphorescence.
In order to understand the mechanism of phosphorescence, we have to understand the excitation process first. In such an excitation process, there is no time lag between the initial absorption of radiation and the step of electronic excitation.
Excitation occurs too rapidly to allow enough time for the orientation of the spin of the electrons to change. This excited electronic state, having two electrons in different orbitals with opposite spins, is called an excited singlet state. If the absorbed radiation is re-emitted almost instantaneously (without any observable time lag), the phenomenon is called fluorescence.
But if there is a significant time lag between the absorption of radiation and its subsequent emission, it implies that a change in the orientation of electronic spin has taken place. This spin inversion gives rise to a lower-energy excited triplet state, in which the two relevant electrons now share the same direction of spin. Any radiation emitted during the transition from this forbidden triplet state back down to the ground singlet state produces what is known as phosphorescence. It is highly probable that the transition into this triplet state is facilitated by an initial loss of vibrational energy through collisions with the exact impurities embedded within the phosphorescent crystals.
Read also: Fluorescence
Test Your Understanding: Phosphorescence Mechanics
1. What primary quantum mechanical event differentiates phosphorescence from standard fluorescence emission?
- (A) Total destruction of the electron cloud
- (B) Inversion of electron spin resulting in a triplet state transition
- (C) Immediate transition directly into the stable ground level without deactivation
- (D) Absorption of multiple low-energy infrared photons simultaneously
View Answer
Explanation: Fluorescence occurs via a spin-allowed singlet-to-singlet transition ($S_1 \rightarrow S_0$). Phosphorescence, however, requires an initial non-radiative spin flip called Intersystem Crossing (ISC) into a triplet state ($T_1$). The subsequent emission ($T_1 \rightarrow S_0$) involves a spin-forbidden transition, making it a much slower process.
2. Why does phosphorescence continue to display an observable glow for seconds, minutes, or even hours after the external light excitation source is turned off?
- (A) Triplet-to-singlet electronic transitions are quantum mechanically 'forbidden' and proceed very slowly.
- (B) Singlet states possess higher total mass than equivalent triplet orientations.
- (C) Aromatic molecules lack clear vibrational relaxation routes.
- (D) The sample absorbs thermal heat from the room and converts it into UV radiation.
View Answer
Explanation: Because transitions changing spin multiplicity (from a triplet state back to a singlet ground state) violate standard selection rules, they have extremely low transition probabilities. This lowers the decay rate drastically, yielding a long emission lifetime.
3. Which of the following substances represents a well-known example that demonstrates persistent phosphorescent behavior?
- (A) Purified liquid Benzene
- (B) Zinc Sulphide (ZnS) doped with heavy metal traces
- (C) Gaseous Sodium vapor
- (D) Distilled Water
View Answer
Explanation: Zinc sulphide along with alkaline earth metal sulphides function as classic phosphors. Trapped impurity zones within their crystal matrix hold molecules in the excited state configuration for long durations, giving them their characteristic glow-in-the-dark properties.