Pumping of nitrogen molecules: As electric discharge is used as pumping source and when electric discharge is passed through the mixture of CO₂, N₂ and He, electrons are accelerated down the tube. These accelerated electrons collide with the N2 molecules and excite them to higher vibrational energy levels. Let us say the N2 excited from level F1 to F2.

The level F2 happens to be Metastable and thus the N2 molecules excited to F2 spend a sufficient amount of time before getting de-excited.

Achievement of population inversion of CO₂ molecules: The excited level (Metastable level) of CO₂ molecules corresponds approximately to the same energy as the energy of the excited level F2 of nitrogen. Thus when N2 molecules in level F2 collide with the CO₂ in the ground state E1, an energy exchange takes place and this result in the excitation of CO₂ molecules to level E5 and de-excitation of N2 to the ground level F1.

Thus population inversion is achieved between vibrational levels E5 and E4 or E5 and E3. Thus E5 is the upper laser level. E3 and E4 are lower laser levels.

Achievement of laser: The following transitions will occur:

E5 to E4 with laser wavelength of 10.6 μm.

E5 to E3 with laser wavelength of 9.6 μm.

Thus, these transitions produce lasers of wavelength 10.6 μm and 9.6 μm which lie in the far infra-red region.

The CO₂ molecules in the states E4 and E3 deexcite to state E2 through inelastic collision with unexcited CO₂ molecules. This process is very fast so there will be accumulation of CO₂ in this level and they can break the population inversion in upper levels because there is probability of excitation of molecules from E2 to E3 and/ or E4.

To stop the accumulation of CO₂ molecules in E2 special additives like He and water vapors are added into the gas mixture. CO₂ molecules return to the ground state E1 through collisions with the He to which it transfers the excitation energy.

Other function of He is to conduct the heat away to the walls keeping CO₂ cold, this is because helium has high thermal conductivity.

Note: In next article, I will explain again why helium is added in CO₂ laser and what is the output and applications of CO₂ laser. In case of any problem in this article or any other physics article, post in the comment section.

Reference: This article is referred from my authored book “optics and lasers” having ISBN 978-81-272-2948-2

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