X-ray Probes of Atoms in Strong-Optical Fields

Linda Young,
Argonne National Laboratory

Abstract: We are investigating how a high-field/ultrafast laser modifies the x-ray photoionization and vacancy decay of an isolated atom. Since many proposed experiments for the next generation x-ray sources, e.g. Linac Coherent Light Source, LCLS, involve laser/x-ray pump–probe techniques on the ?100 fs timescale, it is interesting and important to understand how the x-ray physics of an atom is perturbed due to the presence of a high-power laser. Two effects have been theoretically predicted and observed for valence electrons: 1) a ponderomotive shift of the ionization threshold, and, 2) the appearance of sidebands in photo- and Auger-electron spectra. The shift may be sizable with readily-available laser intensities; at 1014 W/cm2 (1mJ/100ps/10µm2) for 800 nm light, the ponderomotive energy is 6 eV. These shifts have never been observed in the x-ray region or at an inner-shell threshold. Our studies are being conducted at MHATT-CAT at the Advanced Photon Source, where the output of an amplified Ti:sapphire laser system, (~1 kHz, ~1 mJ/pulse), has been overlapped with focused x-ray pulses (106 x-rays/pulse, <10 µm spot size) in an effusive krypton jet. X-ray and ion/electron time-of-flight are used to monitor the x-ray absorption and decay. We are probing the intensity regime from 1013 W/cm2 to ~2 x 1014 W/cm2, where the ionization becomes saturated and dramatic changes to the near-edge absorption spectra are observed. At gas densities ~1013/cm3, presently required for x-ray studies, we find that Coulomb explosion dominates the dynamics of the laser-produced ion/electron ensemble. X-rays are shown to be an excellent spatial and temporal probe of individual ion species in a complex, strong-field environment.