are also interesting because of the strong interactions appearing between the singly-excited and the doubly-excited configur-Rydberg series in strontium found in absorption by selectively laserations.excited atoms.

We describe experimental studies of high-order harmonic generation in rare-gas-like Na+ and K+ ions. The ions were created in a laser-produced plasma and the harmonics were generated with a shortpulse, high-power, titanium-sapphire laser operating at 794 nm. The highest harmonic order observed was the 27th {29. 4 nm), while much higher harmonic orders were observed in a jet of neutral Ne gas under similar conditions. Pronounced ring structures were observed in the harmonic far-field distribution and these structures varied with focusing condition. The cutoff in the harmonic spectra at high orders may be due to ionization-induced defocusing of the fundamental laser beam. Calculations illustrating how this effect reduces the peak intensity obtained in the plasma compared with the intensity obtained in the Ne gas are presented. PACS number(s): 32.80.Rm, 42.65.Ky

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High-power Ti:sapphire laser pulses (150 mJ in 150 fs) were focused into a gas jet of He, N2, CQ2, 02, 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 X 10' %/cm' 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.