Generation of tunable coherent XUV harmonic emission from Xe at low pressure.

We report the harmonic spectral properties of Xe at low gas pressure and the significant amount of redshift and blueshift of the harmonic spectra. Different amounts of harmonic frequency shift are obtained by adjusting the gas pressure and peak laser intensity; namely, by varying the ionization of the gas medium. The observed blueshift shows that nondiabatic effects in the ionized gas medium play a major role, while the observed redshift toward higher gas pressure is attributed to the decrease in electron density effects due to the defocusing and electron-ion recombination. We demonstrate the possibility of coherently tuning the soft x-ray source by taking advantage of the frequency shift of harmonics. [ABSTRACT FROM AUTHOR]