Two-Photon Resonant Third Harmonic Generation in Carbon Monoxide

Tunable coherent extreme ultraviolet radiation has been generated in CO by two-photon resonant third harmonic generation via the A¹ II(v' = 4) ←X¹ Σ⁺(v" = 0 ) transition. The third nharmonic spectrum exhibits resolved and improved rotational lines due to the expansion cooling of the molecular beam. The saturation and the phase-matching of the nonlinear process are also discussed.