3.12 Atomic energy exchanges and physics of gas discharge tubes.  (Page 2/2)

This phosphorescence effect is used in Oscilloscope screens where we want persistence of the image.

In mercury lamps the fluorescence effect is used. In a mercury tube, a vacuum tube contains a drop or two of molten mercury along with inert Argon gas. This is vaporized and a large voltage is applied to cause an electric discharge in mercury vapour. In the process the mercury atoms are ionized. Almost instantaneously ionized atoms neutralize by the process of spontaneous emission and emit Ultra Violet light which is invisible. The UV light strikes the walls of the tube which is coated with fluorescence material called phosphor. This coating gets excited by the absorption of UV light and immediately relaxes to the ground state by emitting soft white light. Here relaxation time is of the order of nanosecond. Hence Mercury Tube is also called Fluorescence Tube. Flourescence is much more efficient than Incandescence used in Incandescent Lamps invented by Thomas Alva Edison [Appendix XXXII ] .

In a similar fashion the discharge in various gases cause spontaneous emission of various colored light.

Mercury Discharge Physics :

Hg(vapor) + collision energy(4.9eV) = Hg* (ionized mercury atoms);

(Collision Energy is caused by highly accelerated ionized particles in presence of critical electric field. The ionized particles are caused due to cosmic particles striking the mercury vapor atoms).

Hg* = Hg + 4.91eV

And λ= 1.24/(4.91eV)= 0.252μm ( Ultra-violet light );

1 ^st ionization energy ( energy required for stripping the outermost electrons) = 2.00eV.

Sodium Vapor Discharge Lamp :

Na + collision energy (2.85eV)= Na*

Na* = Na + 2.85eV photon;

And λ= 1.24/(2.85eV)= 0.435μm ( Yellow light );

First Ionization Energy : E ₁ = 495 KJ/MOLE; (0.93eV)

Second Ionization Energy : E ₂ = 4560 KJ/MOLE;

Third Ionization Energy : E ₃ = 6900 KJ/MOLE;

Fourth Ionization Energy : E ₄ = 9540 KJ/MOLE;

Fifth Ionization Energy : E ₅ = 13400 KJ/MOLE;

Sixth Ionization Energy : E ₆ = 16600 KJ/MOLE;

In first ionization the outermost electrons are being ripped apart from the atom. In second ionization, the second outermost electrons are being ripped apart from the atom. And so on.

Neon Discharge Lamp :

Ne + collision energy(10.6eV) = Ne* Ne* = Ne + Orange-red light ;

E ₁ = 2080, E ₂ =3950, E ₃ =6120, E ₄ =9370, E ₅ = 12200, E ₆ = 15000 (KJ/MOLE);

First ionization energy = 4.84eV

Argon Discharge Lamp :

Ar + collision energy(7.7eV) = Ar*

Ar* = Ar + Bluish Purple ;

E ₁ = 1521, E ₂ =2666, E ₃ =3900, E ₄ =5770, E ₅ = 7240, E ₆ = 8800 (KJ/MOLE);

First ionization energy = 3.2eV

Krypton Discharge Lamp:

Kr + 6.8eV(1400 KJ/Mole) = Kr*

Kr* =Kr + White light

First ionization energy = 2.94eV

Helium Discharge Lamp :

He +12.3eV(2373 KJ/MOLE) =He *

He* =He + ?

E ₂ =5248 KJ/MOLE

Hydrogen Discharge Lamp:

H + 7.18eV = H*

Stimulated Emission Phenomena:

Stimulated Emission is induced emission when incident photon matches the transition energy . This effect is used in LASER( Light Amplification by Stimulated Emission Radiation).

In Fig.(1.19) the spontaneous emission is shown.

In Fig.(1.20) the stimulated mission is shown.

Fig.(1.19). The mechanism of Spontaneous Emission.

Fig(1.20). The mechanism of Stimulated Emission.