June 10, 2003

EXAFS Under Extreme Experimental Conditions: EXAFS in the Realms of Small Spot Size, Low Energy, Low Sample Concentration, or Fast Time Resolution

EXAFS is well established as a measurement technique used in a broad range of scientific disciplines. Within certain experimental constraints, high quality data is routinely obtained by users of synchrotrons around the world. In recent years, the scope of EXAFS has been expanded by advances in measurement techniques. At this
year's NSLS Users’ Meeting, these exciting developments were explored in a workshop entitled, "EXAFS Under Extreme Experimental Conditions: EXAFS in the realms of small spot size, low energy, low sample concentration, and fast time resolution" and organized by Bruce Ravel of the Naval Research Laboratory in Washington, DC. Just as EXAFS is commonly used by researchers from many different scientific disciplines, so too did our speakers present results from many different disciplines, including chemistry, environmental science, and materials physics.

Barukh Yaakobi of the University of Rochester began the workshop by discussing the use of laser-generated shocks with imploding targets as the radiation source for his EXAFS experiments. Dr. Yaakobi discussed measurements of an ultra-fast structural phase transition in titanium metal induced by the laser-generated shock and measured in Dispersive mode. This was followed by Vadim Palshin from Louisiana State
University and CAMD discussing the experimental challenges of low-energy EXAFS measurements. He presented detailed structural refinements on the silicon K-edge silicon-containing, thin, amorphous carbon films.

Lin Chen of Argonne National Laboratory spoke of using the time structure of a stored current to measure photo-excited molecular structures. In these experiments, very short lived molecular states are measured in a pump-probe geometry wherein the molecular population is laser-excited and the excited state is measured by an x-ray pulse incident during its lifetime. Shelly Kelly also of Argonne National Laboratory spoke of uranium L3-edge EXAFS at environmentally relevant concentrations. Environmentally relevant concentrations strain the limits of detectability even with third generation light sources and Dr. Kelly discussed the experimental concerns of low sample concentrations and addressed the limits of sample dilution for full analysis of the EXAFS signal. The final talk was by Ronald Cavell of the University of Alberta. He spoke on the use of microprobe EXAFS sources to map the composition of heterogeneous materials. He presented results of mapping and structural determination of the components of a meteor sample.

There is a rule of thumb that the measurements of elemental identification, XANES measurement, and EXAFS measurements require increasing orders of magnitude of photon flux or sample concentration. Consequently detailed EXAFS analysis in the limits of small spot size, low energy, low concentration, or fast time resolution requires special considerations for sample preparation and measurement. In many cases these experimental limitations have only been addressed since the advent of technical advances such as third generation sources. This workshop provided an excellent snapshot of the current state of the art for each of these extreme realms of EXAFS measurement and analysis.

FOR MORE INFORMATION
Bruce Ravel
Code 6134, Building 3, Room 222
Naval Research Laboratory
Washington DC 20375, USA
Tel.: (202) 767-5947
FAX: (202) 767-1697
Email: ravel@phys.washington.edu

NRL Synchrotron Radiation Consortium (NRL-SRC)
Beamlines X11a, X11b, X23b, X24c, U4b
National Synchrotron Light Source
Brookhaven National Laboratory, Upton, NY 11973