Energy-level diagrams for atoms that comprise a helium-neon laser are given below. As indicated on the left, the helium atom is excited by an electrical discharge and an electron jumps from energy level E₀ to energy level E₂.

 

 

The helium atom (atomic mass 4) then collides inelastically with a neon atom (atomic mass 20), and the helium atom falls to the ground state, giving the neon atom enough energy to raise an electron from E₀^' to E₂^'. The laser emits light when an electron in the neon atom falls from energy level E₂^' to energy level E₁^'.