Rami Arieli: "The Laser Adventure" Chapter 6, Helium-Neon Lasers, page 2

The role of the Helium gas in He-Ne laser
The role of the Helium gas in He-Ne laser is to increase the efficiency of the lasing process. Two effects make Helium particularly valuable:

1. The direct excitation of Neon gas is inefficient, but the direct excitation of He gas atoms is very efficient.
2. An excited state of the He atom (labeled E₅) has an energy level which is very similar to the energy of an excited state of the Neon atom (also labeled E₅).

The excitation process of the Neon atoms is a two stages process:

• The high voltage causes electrons to accelerate from the cathode toward the anode. These electrons collide with the He atoms and transfer kinetic energy to them.
• The excited Helium atoms collide with the Neon atoms, and transfer to them the energy for excitation.

Thus Helium gas does not participate in the lasing process, but increases the excitation efficiency so that the lasing efficiency with it increase by a factor of about 200 (!).
If you want to play with an interactive simulation about the energy level diagram of a He-Ne Laser, click here. (VQM Simulation - He-Ne)(Requires Shockwave).

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