Abstract
Table-top laser-driven hard x-ray sources with kilohertz repetition rates are an attractive alternative to large-scale accelerator-based systems and have found widespread applications in x-ray studies of ultrafast structural dynamics. Hard x-ray pulses of 100 fs duration have been generated at the Cu Kα wavelength with a photon flux of up to 109 photons per pulse into the full solid angle, perfectly synchronized to the sub-100-fs optical pulses from the driving laser system. Based on spontaneous x-ray emission, such sources display a particular noise behavior which impacts the sensitivity of x-ray diffraction experiments. We present a detailed analysis of the photon statistics and temporal fluctuations of the x-ray flux, together with experimental strategies to optimize the sensitivity of optical pump/x-ray probe experiments. We demonstrate measurements close to the shot-noise limit of the x-ray source.
Highlights
The precise determination of the ground state electron density q(r) of matter in the solid state by x-ray diffraction and its analysis have developed into a mature field and provide detailed insights into the nature of chemical bonding in a great variety of materials.1,2 The study of transient electron density q(r, t), gained from femtosecond x-ray diffraction employing an optical pump/x-ray-probe setup, is an exciting recent development
We present a detailed analysis of the photon statistics and temporal fluctuations of the x-ray flux, together with experimental strategies to optimize the sensitivity of optical pump/x-ray probe experiments
Ultrafast x-ray diffraction experiments based on pump-probe schemes measure pumpinduced changes in the x-ray flux diffracted from the probe pulse on particular Bragg reflections and/or Debye-Scherrer rings
Summary
The precise determination of the ground state electron density q(r) of matter in the solid state by x-ray diffraction and its analysis have developed into a mature field and provide detailed insights into the nature of chemical bonding in a great variety of materials. The study of transient electron density q(r, t), gained from femtosecond x-ray diffraction employing an optical pump/x-ray-probe setup, is an exciting recent development. The study of transient electron density q(r, t), gained from femtosecond x-ray diffraction employing an optical pump/x-ray-probe setup, is an exciting recent development It allows for atomic motion or relocation of electronic charge, both relevant in chemical or physical processes, to be resolved on atomic time and length scales.. The required femtosecond hard x-ray pulses have been generated by accelerator-based sources, such as free electron lasers or slicing schemes at synchrotrons, and by laser-driven table-top femtosecond hard x-ray sources The latter offer a moderate hard x-ray flux with a negligible timing jitter between pump and probe pulses, long-term access for lab-based experiments, and comparably low cost for implementation.. The generated x-ray flux displays fluctuations on various time scales, requiring efficient normalization and averaging methods to measure small changes in the diffracted intensity While these issues require an optimization of the experimental parameters, a fundamental limit consists in the photon counting statistics. The results provide deeper insights into the characteristic spatial and temporal fluctuations of the hard x-ray source
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