Two- and Three-P hoton Resonant Third Harmonic Generation in Molecular Nitrogen

Tunalbe coherent extreme ultraviolet radiation has been produced in nitrogen by third harmonic generation (THG) processes enhanced by two-photon resonance with the a¹II_g(v' = 1) state and three-photon resonance with the c'₄ ¹Σ⁺_u(v' = 1) state, respectively. The highly resolved THG spectra imply that THG can be used to probe the higher excited states and to detect the rotational population of a molecular gas.