Soft x-ray pulse-length measurement by pump-probe absorption spectroscopy

We have demonstrated a system for subpicosecond, soft-x-ray continuum, pump-probe absorption spectroscopy. Using multiphoton ionization to abruptly change the x-ray absorption spectra of a gas, we have measured the temporal profile of laser-generated x-ray pulses near 90 eV. Although the x-ray pulses from a laser-generated plasma here were only as short as approximately 20 psec, the technique is extendible to higher energy x rays that will have pulse durations approaching 100 fsec. We also present the absorption spectrum of Kr ions produced under conditions of high intensity non-resonant multiphoton ionization. The spectra are identified with a fit to the Cowan code.