Speaker: Jan M. Feldkamp, SLAC/LCLS
Program Description
Small- and wide-angle x-ray scattering experiments have been extremely successful in investigating structural properties in a variety scientific fields. Typically, a one-dimensional radial pair-correlation function is calculated to study properties like nearest-neighbor distances and derive structural information. In this traditional analysis, information present in the second, angular dimension of the two-dimensional detector image is essentially discarded by averaging over all angles. The technique of x-ray cross-correlation analysis (XCCA) makes use of this second dimension by calculating an angular correlation function from two-dimensional scattering patterns. Conceptually going back to work by Kam some 35 years ago, interest in the x-ray community was sparked by a recent experiment performed at the synchrotron radiation source ESRF by Wochner et al., who were able to reveal local symmetries in a colloidal glass.
Using the LCLS as a light source allows us to extend the technique to fast dynamic systems, by virtually "freezing" the molecular arrangement in time due to the short x-ray pulse duration on the femtosecond time scale. We study one of nature's most fundamental dynamic systems, liquid water. Water, despite its simple molecular composition, displays a variety of extraordinary macroscopic properties. Many of the anomalies that distinguish water from other liquids are not fully understood to date, but are believed to be connected to structural properties of the hydrogen-bonded network of water molecules.
In my talk, I am going to introduce the technique of angular x-ray cross-correlation analysis, describe the recent experiment on liquid water at the
CXI instrument of LCLS and report on the current status of the data analysis efforts.
