Coherence Conversion for Femtosecond X-ray Spectroscopy with 100-Picosecond Synchrotron Radiation and a Slow Detector

The elementary electronic processes of chemistry and condensed-matter physics take place at the femtosecond time scale and the sub-nm length scale of chemical bonds. X-ray spectroscopy offers information that is complementary to laser-based measurements, in particular with regard to laser-induced changes in the chemical bonds of specific elements in the sample, and to re-distribution of charge carriers within crystal unit cells. However, in direct laser-pump, x-ray-probe measurements, the time resolution is limited by the duration of the x-rays, or the time resolution of the detector, as well as the synchronisation of the pump laser pulses to the x-rays. Therefore, synchrotrons with their, typically, 100-ps-long pulses cannot directly yield femtosecond resolution, even with the fastest detectors (streak cameras) that resolve down to about 1 ps. However, by converting the relatively good transverse (spatial) coherence of synchrotron radiation to become effective for the longitudinal (time) domain, a femtosecond resolution over a range of hundreds of femtoseconds can be achieved.

The gain in doing so can be understood by realizing that, without a femtosecond source like a free-electron laser, x-rays cannot be controlled on femtosecond time scales. In contrast to this, x-rays can easily be controlled on micron scales (setting slits, etc.), which correspond to femtoseconds at the speed of light. The spatial domain can be applied to the time domain by the geometry of laser/x-ray incidence directions followed by far-field diffraction. In a more abstract description to establish a broader context, this is a sequence of phase-space transformations that rotate from space to time. Data from a test experiment will be presented [2].

[1] manuscript accepted at J. Modern Optics
[2] manuscript in preparation