Luminescence, Fluorescence and Phosporescence

LUMINESCENCE, FLUORESCENCE AND PHOSPORESCENCE

When the atoms in the solid is excited by some means (optical, electrical excitation etc.) and if the atoms return from excited state to the ground state by emitting light, (Fig. 4.9) it is called Luminescence.

From Fig. 4.9 we can see that the difference in energy is emitted in the form of light.

i.e. E₂-E₁= hv =hc / λ

Wavelength of light emitted 

Note: Since E₁ and E₂ represents the group of energy levels, instead of emitting a single wavelength, a band of wavelengths are emitted.

Based on their excitation, we can classify the luminescence into many types viz..

(i) Photo-luminescence: Here the excitation of atom occurs due to photons (or) light.

When the atoms are excited by incident photons and if the atom returns from excited state to ground state by emitting photons it is called photoluminescence. 

(ii) Electro-luminescence: Here the excitation of atoms occur due to electric field.

When the electrons are excited from valence band to conduction band by applying d.c. (or) a.c. field and if they emit light during deexcitation it is called Electro-luminescence.

(iii) Cathodo-Luminescence: Here the excitation of atoms occurs due to electron bombardment.

When high energetic electron beam (external beam from hot catode) hits the solid, the electron-hole pairs are created in the solid. During the recombination of electron-hole pairs the light is emitted. This process of emission of light is called Cathodo-luminescence.

(iv) Thermo-luminescence: Here the excitation of atoms occurs due to heating.

When the atoms are excited due to the supply of thermal energy and if they emit light during deexcitation, it is called Thermo-luminescence.

(v) Chemi-luminescence: Here the excitation of atoms occurs due to chemical reactions.

When the atoms are excited due to some chemical reactions and if they emit light during deexcitation process, it is known as Chemi-luminescence

 (vi) Injection luminescence:

Here due to the application of forward biasing to the p-n junction diode, the light is emitted from the junction due to recombination of electrons and holes. When a p-n-junction diode is forward biased, the majority charge carries moves from n to p and p to n region and becomes minority carriers. Then these excess minority charge carriers diffuses and combines with the majority charge carriers in n and p regions to emit light and is called Injection Luminescence. This is the principle used in Light Emitting Diode (LED).

(vii) Fluorescence

When the time taken by an atom to make the transition from E1 to E2 (excitation) is equal to the time for which the light emission (luminescence) persists, then it is called fluorescence.

(viii) Phosphorescence

When the time taken by an atom for excitation is less compared to the time for which the luminescence persists it is called phosphorescence.

Explanation: When the atoms are excited by some means, it is not so that all the atoms will return from excited to ground state directly, rather, they spend some time in the traps (Energy state which arises due to impurity atoms).

The time during which an atom spends in the trap depends on two factors viz., (i) Depth of the trap and (ii) The temperature.

So, if an atom spends more time in a deep trap at low temperature, then the luminescence persists for a longer time which is called phosphorescence (or) after glow.