You work for a company that makes red helium?neon lasers widely used in physics experiments. Figure 36.19 shows an energy-level diagram for this laser. An electric current excites helium to a metastable level E₁ at 20.61 eV above the ground state. Collisions transfer energy to neon atoms, exciting them to E₂ = 20.66 eV. The lasing transition drops the atoms to E₃, emitting a 632.8-nm photon in the process. You?re asked to find the maximum possible efficiency for this laser?that is, the light energy emitted as a percentage of the energy supplied to excite the atoms.

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22 E, E, 632.8 nm 20 18 - 16 - 14 - 12 - 10 - Eg Helium E Neon 6. 4) 2. Energy (eV abo ve common ground state)