The U.S. Department of Energy's Advanced Photon Source (APS) at Argonne National Laboratory, and the APS Users Organization announced that the 2009 Arthur H. Compton Award will be presented jointly to Gerhard Grübel, Simon Mochrie, and Mark Sutton for their pioneering efforts in x-ray photon correlation spectroscopy (XPCS), which exploits the coherent properties of synchrotron x-rays to study the slow dynamics of condensed matter at short length scales.

"XPCS seemed like a heroic experiment only a decade ago, but it is now used routinely to do great science at APS and other sources," said Murray Gibson, director of the APS. "We are grateful for the pioneering vision of our winners in making this research possible."

The XPCS technique has evolved into a sophisticated tool for studying slow dynamics in inhomogeneous systems at length scales too small for other techniques. The wide range of systems studied includes block copolymers, micellar systems, colloidal suspensions, liquid surfaces, molten polymer films, membranes, and binary alloys. The award winners have played a significant role in driving the evolution and application of this technique.

X-ray photon correlation spectroscopy makes use of the way coherent light scatters from irregular structures. The coherent portion at the center of a synchrotron undulator beam is selected with a pinhole aperture. When this coherent beam hits the sample, the scattered light bunches into spots, called speckles. As the structure of the sample changes, the intensity of light at each speckle changes. By monitoring how the intensity fluctuates across the whole pattern of speckles—like watching flames pass through a bed of embers—it is possible to learn how the structure of the sample changes with time.

The technique was first reported for x-rays by Sutton, Mochrie, and colleagues in a 1991 letter to Nature [1], in which they described work at the National Synchrotron Light Source at Brookhaven National Laboratory. A similar technique, dynamical light scattering with visible laser light, was widely used at the time, but the extension to x-rays makes it possible to study opaque samples and work at much smaller length scales (well under 200 nanometers).

The technical challenges of the technique were significant. Because so little of the available beam was coherent, and because x-rays interact weakly with samples in general, these XPCS pioneers had to devise both efficient detection strategies for tiny signals and sophisticated algorithms to analyze those signals as they varied over a large area of detector space.

Simon Mochrie and Mark Sutton were key contributors in the development of beamline 8-ID at the APS, which remains the only facility for XPCS in the United States. Both were active in the management and scientific direction of the IBM-McGill-MIT-Yale Collaborative Access Team (IMMY-CAT), which built and operated Sector 8 until it transitioned to APS management in 2003. Many other investigators associated with IMMY-CAT also contributed to the development of the technique and its application. Mochrie is currently Professor of Physics and Professor of Applied Physics at Yale University, New Haven, Connecticut; Sutton is Professor of Physics at McGill University in Montreal, Canada.

Gerhard Grübel brought the technique to Europe, leading the development of the ID10 (Troika) beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and pioneered many innovative applications of XPCS. He is now a senior scientist at HASYLAB at the Deutsches Elektronen-Synchrotron in Hamburg, Germany, where he is involved in the development of coherence based techniques for free-electron laser sources.

In addition to developing the "how" of XPCS, all three winners have used it to significantly advance the "what" of their own disciplines. The following are examples selected by the winners as illustrating the impact of XPCS in their work.

Simon Mochrie studies the properties, phase behavior, and phase transitions of soft matter. His work to characterize a sponge phase of a block copolymer [2] marked a step forward in terms of both technique and science. The quality of the data showed that XPCS is useful for studying polymer dynamics; furthermore, as one of the first studies to use a fast x-ray area detector, this work has prompted further development of those detectors. In terms of the system studied, XPCS revealed a previously unobserved crossover from stretched to compressed exponential relaxations as a function of temperature. The data also provided a clear example of mysterious compressed exponential relaxations that do not appear to evolve with time, realizing a new paradigm for relaxations in highly viscous media.

Mark Sutton's group studies the time evolution of non-equilibrium systems. Mark and colleagues have since demonstrated a controllable way to combine a reference signal (heterodyning) with coherent small angle x-ray scattering [3]. This adaptation of the XPCS technique gives direct access to phase information that can, for instance, be used to separate the effects of advection (local flow) from dissipation (randomness).

Gerhard Grübel and his group work primarily on the bulk and surface dynamic properties of complex fluids and more recently on glassy and magnetic systems. His work on colloidal silica suspensions [4] illustrated the strength of XPCS in combination with small-angle scattering for the quantitative characterization of colloidal fluids. It motivated a series of subsequent studies challenging, in particular, our knowledge of the direct and indirect hydrodynamic interactions in concentrated soft-sphere fluids.

Along with their early collaborators Steve Dierker, Larry Lurio, Ian Robinson, Brian Stephenson, and others, the three award winners have pioneered a scientific thrust that has had long-range influence on research at synchrotron sources. The techniques they have developed will continue to drive development and science at beamlines at APS, ESRF, and elsewhere and will have significant influence on the next generation of free-electron lasers.

The Arthur H. Compton Award was established in 1995 by the APSUO to recognize an important technical or scientific accomplishment at, or beneficial to, the APS.

Compton was an American physicist who won the Nobel Prize for Physics in 1927 for discovering and explaining changes in x-ray wavelengths resulting from x-ray collisions with electrons, the so-called Compton effect. This important discovery in 1922 confirmed the dual nature (wave and particle) of electromagnetic radiation. A Ph.D. from Princeton University, Compton held many prominent positions including professor of physics at The University of Chicago and chairman of the committee of the National Academy of Sciences that studied the military potential of atomic energy. His position on that committee made Compton instrumental in initiating the Manhattan Project, which created the first atomic bomb.

References

[1] M. Sutton, S. G. J. Mochrie, T. Greytak, S. E. Nagler, L. E. Berman, G. A. Held, and G. B. Stephenson, "Observation of speckle by diffraction with coherent X-rays," Nature 352, 608 (15 August 1991). DOI:10.1038/352608a0

[2] P. Falus, S. Narayanan, A. R. Sandy, S. G. J. Mochrie, "Crossover from stretched to compressed exponential relaxations in a polymer-based sponge phase," Phys. Rev. Lett. 97, 066102 (2006). DOI:10.1103/PhysRevLett.97.066102

[3] F. Livet, F. Bley, F. Ehrburger-Dolle, I. Morfin, E. Geissler, and M. Sutton, "X-ray intensity fluctuation spectroscopy by heterodyne detection," J. Synchrotron Rad. 13, 453 (2006). DOI:10.1107/S0909049506030536

[4] G. Grübel, D. L. Abernathy, D. O. Riese, W.L. Vos, and G. H. Wegdam, "Dynamics of dense, charge-stabilized suspensions of colloidal silica studied by correlation spectroscopy with coherent X-rays," J. Appl. Cryst. 33, 424 (2000). DOI:10.1107/S0021889800099994

The Advanced Photon Source at Argonne National Laboratory is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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