Characterization of a pulsed, supersonic gas jet by absorption of high-order harmonics in the extreme ultraviolet spectral range

Abstract

We present a method for determining the temporal and spatial evolution of a gas jet generated by a pulsed nozzle using high-order harmonics of a titanium–sapphire laser. This radiation in the extreme ultraviolet spectral range (17–40 nm) is transmitted through the gas jet and becomes partially absorbed depending on its wavelength and the gas density. If the absorption in this spectral range shows a sufficiently strong dependence on the wavelength, as is the case for many gases including the noble gases argon, neon, and helium, it is possible to select a proper harmonic exhibiting an absorption strong enough to generate a detectable decrease of the transmitted light but still weak enough to allow a significant amount of radiation to be transmitted through the gas jet. In the case of radial symmetry the density profile can be reconstructed by means of the Abel inversion. We show that this method allows for the determination of argon neutral densities as low as 10^{17} cm^{-3} and is also suited for other gases, such as neon and helium.