Quantitative analysis of third harmonic generation of XUV radiation in a cell and a cw free jet of carbon monoxide

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Abstract
In the theory, the two-photon resonant third-order nonlinear susceptibility of a molecular gas is calculated. The variations of this susceptibility, with the rotational quantum numbers of the resonant states, are given in the case of two singlet states of different symmetries, Σ and Π. In the experiment, third harmonic generation of XUV light (94.5–97.5 nm) is obtained with and without two-photon resonance. The gaseous nonlinear medium is formed by CO as used either in a cell, or in a continuous supersonic free jet zone of silence. The intensity of the third harmonic wave generated is well related to the laser power, phase-matching conditions, two-photon resonances, and rotational temperature, through the nonlinear susceptibility, as described theoretically.