1. NSLS-II Friday Lunchtime Seminar

    "Towards studying dynamics over 18 decades with correlation spectroscopy – first XPCS results from European XFEL"

    Presented by Felix Lehmkuehler, Coherent X-ray Scattering Group, DESY, Germany

    Friday, April 5, 2019, 12 pm
    NSLS-II Bldg. 743 Room 156

    Hosted by: Ignace Jarrige

    Many phenomena in soft matter physics and biology happen on microsecond timescales, e.g., folding kinetics of proteins, where the time scale is basically defined by the protein size. Despite of the importance of such processes, μs time scales are very difficult to access in X-ray scattering experiments at storage rings as well as in quasi-elastic neutron scattering. The time structure of the European XFEL with MHz pulse repetition rate in a bunch train enables for the first time structural studies of dynamics and kinetics at such time scales. A method to investigate such phenomena is X-ray photon correlation spectroscopy (XPCS), tracking changes of the coherent diffraction pattern, the so-called speckle pattern, over time. XPCS has become a well-established technique at modern storage ring sources studying timescales ranging from several 1000 s down to milliseconds. At FEL sources, the higher average flux and superior degree of coherence allow the access to shorter timescales down to femtoseconds using split-pulse techniques. On the other hand, sequential-pulse XPCS is limited by the repetition rate of the FEL pulses, i.e. about 8 to 50 ms at normal-conducting hard X-ray FEL sources such as LCLS (USA) and SACLA (Japan). The high repetition rate of the European XFEL enables dynamics measurement on sub-μs time scales, which is otherwise difficult to achieve at storage ring, neutron, pump-probe or lab-based experiments. In this talk, first dynamics measurements at FEL sources using correlation techniques will be introduced and recent results on prototypical soft matter samples as well as fs dynamics of liquid water will be discussed. Afterwards, results from our recent XPCS experiment at European XFEL will be discussed, where we successfully performed measurements of (sub-)µs dynamics of soft matter samples. Furthermore, special attention is paid on shot-to-shot and train-to-train fluctuations of coherence and beam pointing obtained by c