Interaction of Ultrashort X-ray Pulses with Material

Abstract: Radiation damage limits the resolution in imaging experiments. Damage is caused by energy deposited into the sample during exposure. Ultrashort and extremely bright X-ray pulses from free-electron lasers (FELs) offer the possibility to outrun key damage processes, and temporarily improve radiation tolerance. Theoretical models indicate that high detail-resolutions could be realized on non-crystalline samples with very short pulses, before plasma expansion.Studies presented here describe the interaction of a very intense and ultrashort X-ray pulse with material, and investigate boundary conditions for flash diffractive imaging both theoretically and experimentally. In the hard X-ray regime, predictions are based on particle simulations with a continuum formulation that accounts for screening from free electrons.First experimental results from the first soft X-ray free-electron laser, the FLASH facility in Hamburg, confirm the principle of flash imaging, and provide the first validation of our theoretical models. Specifically, experiments on nano-fabricated test objects show that an interpretable image can be obtained to high resolution before the sample is vaporized. Radiation intensity in these experiments reached 10^14 W/cm^2, and the temperature of the sample rose to 60000 Kelvin after the 25 femtosecond pulse left the sample. Further experiments with time-delay X-ray holography follow the explosion dynamics over some picoseconds after illumination.Finally, this thesis presents results from biological flash-imaging studies on living cells. The model is based on plasma calculations and fluid-like motions of the sample, supported by the time-delay measurements. This study provides an estimate for the achievable resolutions as function of wavelength and pulse length. The technique was demonstrated by our team in an experiment where living cells were exposed to a single shot from the FLASH soft X-ray laser.