We describe the application of a tunable differential interferometer to the characterization of pulsed gas valves, operating in the low-pressure regime ( Torr). The spatial profile of the pressure in the gas jet has been studied for piezoelectric and electromagnetic valves in various experimental conditions, for both Ne and Ar gases. Moreover the time response of the valves has been investigated by using, for the first time to our knowledge, the third harmonic generation process. The number of third harmonic photons has been determined as a function of the delay time between a Nd:YAG pump laser pulse and the opening time of the valve, thus allowing the determination of the jet temporal profile. The time dependence of the local pressure in the jet has been studied for various gas pulse durations.

We describe here a source of subpicosecond, coherent, partially tunable radiation operating at 500 Hz repetition rate in the VUV region (200−100nm), based on a Ti:S pump laser delivering 500 μJ, 130 fs pulses and producing harmonics both in Xe and in Ar gas targets. The gas target is injected in the interaction chamber through a high repetition rate piezoelectric valve. In particular, we have analyzed the 3rd (265 nm) and the 5th (159 nm) harmonic yields versus laser intensity in different experimental conditions. We also present a study of the optimization of the phase-matching conditions in the gas medium. In virtue of its coherence, partial tunability and high repetition rate characteristics, i.e. short data acquisition times, this laser source lends itself as a very promising tool for carrying out atomic physics experiments in the VUV spectral range.

In harmonic generation experiments carried out at intensities above 1013 W cm-2, the effect of ionisation becomes significant. The presence of free electrons introduces a space-time dependent change of the refractive index of the medium. The important role of the medium ionisation has been recently discussed in ref. 1,2,3 by including in the phase-matching integral the contribution of the electrons to the refractive index.