Splitting FEL X-ray pulses enables measuring ultrafast dynamics

Schematic of the split-pulse experiment using the split-and-delay instrument developed at DESY. Ultrashort X-ray FEL pulses are split into two equal intensity pulses and delayed prior to their arrival at the sample position. The speckle patterns from the sample are collected during the same exposure time of the 2D detector. The resulting speckle contrast will decrease if the sample evolves on the time scale defined by the separation of the two pulses Δt.

In this study splitting ultra-short LCLS pulses enabled measurements of nanosecond dynamics at nanoscales in a colloidal liquid, demonstrating the first split-pulse X-ray Photon Correlation Spectroscopy performed at hard X-ray FEL sources.


Split-pulse experiment was carried out at X-ray Correlation Spectroscopy (XCS) instrument of LCLS. The split-and delay system was used to modify the intrinsic pulse structure of LCLS and changed the time spacing between the FEL pulses from 8.3 ms to 1.3 ns. The sample, i.e., a suspension of gold nanoparticles in hexane was designed to show Brownian motion dynamics at the nanosecond time scales. The split-pulse speckle patterns were collected by the pnCCD detector and the speckle contrast was measured as a function of the scattering vector q at fixed time delay (1.3 ns). Crucial in this study was the extraction of the speckle contrast values from very low intensity FEL speckle patterns by applying new statistical analysis tools (i.e., maximum likelihood fitting) that allows global fitting of all (thousands) collected patterns simultaneously.


This split-pulse X-ray Photon Correlation Spectroscopy experiment paves the way for studing more complex systems at atomic scales including fluctuations in liquid metals, multi-scale dynamics in water, heterogeneous dynamics about the glass transition, and atomic scale surface fluctuations.


Nature Communications 9, 1704 (2018)


DESY Photon Science Highlight