Femtosecond Dynamics in Water and Biological Materials with an X-Ray Laser

Sammanfattning: Using high intensity ultrashort pulses from X-ray free electron lasers to investigate soft matter is a recent and successful development. The last decade has seen the development of new variant of protein crystallography with femtosecond dynamics, and single particle imaging with atomic resolution is on the horizon. The work presented here is part of the effort to explain what processes influence the capability to achieve high resolution information in these techniques. Non-local thermal equilibrium plasma continuum modelling is used to predict signal changes as a function of pulse duration, shape and energy. It is found that ionization is the main contributor to radiation damage in certain photon energy and intensity ranges, and diffusion depending on heating is dominant in other scenarios. In femtosecond protein crystallography, self-gating of Bragg diffraction is predicted to quench the signal from the latest parts of an X-ray pulse. At high intensities ionization is dominant and the last part of the pulse will contain less information at low resolution. At lower intensities, displacement will dominate and high resolution information will be gated first. Temporal pulse shape is also an important factor. The difference between pulse shapes is most prominent at low photon energy in the form of a general increase or decrease in signal, but the resolution dependance is most prominent at high energies. When investigating the X-ray scattering from water a simple diffusion model can be replaced by a molecular dynamics simulation, which predicts structural changes in water on femtosecond timescales. Experiments performed at LCLS are presented that supports the simulation results on structural changes that occur in the solvent during the exposure.