Third harmonic generation (THG) in atomic vapor is a well-known source of VUV light.1 Because the third-order susceptibility is low, the pump light intensity must be high, and often competing effects such as multiphoton ionization (MPI) and dielectric breakdown occur. Additionally, for molecules dissociation may also occur. Thus molecules have only rarely been studied for this purpose.2 Carbon monoxide (CO), however, is an attractive candidate for THG due to its high dissociation energy, 11.11 eV, and Ionization potential, 14.01 eV. For the v′ = 0, 1, and 2 members of the A1∏-X1∑ progression, a 3-photon resonance is present below the dissociation energy, and 6 pump photons are required to reach the ionization potential.
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