XUV SPECTRA OF OPTICAL-FIELD-IONIZED PLASMAS

High-power Ti:sapphire laser pulses (150 mJ in 150 fs) were focused into a gas jet of He, N2, CO2, O2, SF6, or Ar emitted from a pulsed nozzle. The xuv spectra that were generated are reported and analyzed. For all gases the spectra show strong lines corresponding to single-electron transitions. This indicates that the applied intensities of up to 5×1016 W/cm2 readily strip the atoms of all outer electrons, with three-body recombination populating excited levels of the next lower ionization stage. Simulations of the plasma evolution after the laser pulse were performed for helium and nitrogen. In the case of helium, good agreement with the experiment can be obtained for specific initial conditions. For nitrogen, mechanisms other than three-body recombination must be invoked to explain the experimental spectra. Gain measurements were made by comparing longitudinal and transverse spectra. Gains on lines connecting to the ground state or to a quasi-ground-state were not observed, but indications of gains on lines between excited states are reported. © 1995 The American Physical Society.