Describe how the laser transitions are arisen.

The given energy level diagram could describe how the laser transitions arise.

Collisions in between the He atoms and Ne atoms because of excitations. The 2s states (as E3 and E5) of He are meta stable, therefore collisions of He atoms into this high state cause excitations into the Ne atoms. At last, the stimulated emission takes place in between E5 and E2leading to the emission of wavelength as:

ΔE = 20.66 - 18.70 = 1.96eV

that is  Λ = hc/ΔE

= (6.63 × 10^-34 × 3.0 × 10⁸)/(1.6 × 10^-19 v 1.96)

= 632.8nm