Question

how population inversion is achieved in the he/nesystem?

Answer 1

Because energy is transferred from the excited helium atoms to the neon atoms, which produce the light we see… Which is then optically amplified.

The mechanism producing population inversion and light amplification in a HeNe laser plasma originates with inelastic collision of energetic electrons with ground state helium atoms in the gas mixture. As shown in the accompanying energy level diagram, these collisions excite helium atoms from the ground state to higher energy excited states. Because of a fortuitous near coincidence between the energy levels of the two He metastable states, and the levels of neon, collisions between these helium metastable atoms and ground state neon atoms results in a selective and efficient transfer of excitation energy from the helium to neon.