Femtosecond optical/hard x-ray timing diagnostics at an FEL: Implementation and Performance

摘要

The development of Free Electron Lasers has opened the possibility to investigate ultrafast processes using femtosecond hard x-ray pulses. In optical/x-ray light pump/probe experiments, however, the time resolution is mainly limited by the ability to synchronize both light sources over a long distance (＞100 fs FWHM) rather than their pulse length (＞10 fs FWHM). We have implemented a spectrally encoding x-ray to optical laser timing diagnostic into the XPP beamline at LCLS with a timing uncertainty down to 10 fs. An x-ray induced change of refractive index in a solid target is temporally probed for single pulses by a chirped white light pulse. By resorting single shot data to the timestamps obtained by the diagnostics, the temporal data quality can be improved to basically pulse length limited time resolution. By interchangable targets and adjustable x-ray and laser foci, the method was successfully applied for very different x-ray parameters. These are different photon energies in the range of 6-20 keV, which at LCLS also includes application of 3rd Harmonic radiation, pulse energy, and bandwidth, when using a Si(111) monochromator.