Principle of (He-Ne) Laser : Here Helium atoms don’t produce direct laser photons, but instead, the Helium atom is excited by an electric field. Then Helium atoms will collide with Neon atoms while traveling inside the Laser chamber/tube. When it does, then the Neon atom will collide with Helium atoms. In every collision with Neon atoms, energy is transferred. Thus Neon atoms are excited to a metastable state. When a sufficient number of Neon atoms reaches this state of population inversion, hence the lasing can take place as shown in the below figure.

My question is how do we know this phenomenon occurs in order?. Why can’t the Neon atom be excited by collision with the wall of the chamber/tube or electric field and then transfer energy to Helium atoms and thus metastable state and hence lasing occurs with Helium?

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  • $\begingroup$ I'm not sure how this was understood. It certainly took a lot of specialized knowledge. But once proposed it can be verified by various means, for example observing laser action and other spectral features as the He:Ne ratio is changed. $\endgroup$
    – garyp
    Nov 6, 2021 at 14:26
  • $\begingroup$ I am glad you could understand where I got stuck in physics. $\endgroup$ Nov 6, 2021 at 14:56
  • $\begingroup$ Can someone explain the order of collision and energy transfer? $\endgroup$ Nov 7, 2021 at 12:28

1 Answer 1


I think what I will be saying is true, but I might be wrong.

So, the E-field creates a plasma of the medium. This allows the creation of free electrons, which can accelerate in the electric field, and when colliding with He atoms, excite the electrons to the 2s shell.

These do not easily radiate because its a transition between two $S$ shells, so $\Delta J=0$, meaning its a "forbidden transition".

They can however transfer their energy via collisions with neutral Ne atoms like the diagram above.

And from there, its like they say.


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