Third harmonic generation of extreme ultraviolet radiation in a nitrogen continuous free jet zone of silence

Abstract

The interpretation of third harmonic generation, as obtained by crossing and focusing a pulsed laser beam on the axis of a continuous supersonic jet of nitrogen, is discussed and treated quantitatively. The geometry of the laser modes, as well as the inhomogeneity of the free jet zone of silence surrounded by the shock wave structure and the background gas, are taken into account. In this way, the variation of the phase matching factor is calculated in terms of the axial distance from the nozzle to the laser focus. This makes it possible to obtain the free jet characteristics from careful measurements of two-photon resonant, or nonresonant, frequency conversion rates. Thus the variations along the jet axis of the number density and the rotational temperature (decreasing down to 6 K), are obtained in good agreement with theoretical predictions.