Scientists capture first-ever X-ray laser image using a single, ultra-fast shot

Scientists have for the first time used an extremely short and intense X-ray laser pulse to obtain a high-resolution image of a nanoscale object before the laser destroyed the sample. The collaboration, which included researchers from the Department of Energy's Stanford Linear Accelerator Center (SLAC), also set a speed record of 25 quadrillionths of a second (25 femtoseconds) of the duration of the X-ray pulse used to acquire the image. The results are published in the Nov. 12 online edition and December print edition of Nature Physics.

"This result is a remarkable validation of the concept of imaging using single pulses from a free-electron laser," said Keith Hodgson, director of photon science at SLAC and a co-author of the paper. "This is just the first glimpse of the breakthrough discoveries that will come from the Linac Coherent Light Source [LCLS] when it becomes operational in 2009."

Free-electron lasers are a new class of lasers that create extremely intense photon pulses in the X-ray spectrum using a beam of electrons from a particle accelerator. When fully operational, the LCLS free-electron laser will use SLAC's existing linear accelerator, or "linac," to produce laser light that is 10 billion times brighter than any other X-ray source on Earth.

The technique used to capture the image is called "flash diffraction imaging." Researchers say that the experiment demonstrates the principle behind atomic-scale flash imaging that will be applied when even more powerful X-ray free-electron lasers are available, such as the LCLS, now under construction at SLAC; the SPring-8 SCSS facility in Japan; and the European XFEL in Hamburg. According to researchers, these lasers will give scientists unprecedented insight into materials science, chemistry, biology, medicine and other fields.

Computer models had suggested that by precisely tuning an X-ray laser, images of microscopic and even atom-sized objects could be obtained in the fraction of a second before the sample is stripped of its electrons and destroyed. But until now there had been no experimental verification of this principle.

The experiment was conducted by an international team led by SLAC Professor Janos Hajdu, who also is affiliated with Uppsala University in Sweden, and Henry Chapman of Lawrence Livermore National Laboratory (LLNL). Using the free-electron laser at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, the research team zapped a sample that contained nanometer-sized objects and recorded the pattern of scattered X-rays—the diffraction pattern—before the laser destroyed the sample. A special computer algorithm was then used to recreate an image of the object based on the recorded diffraction pattern.

The DESY laser used in the experiment generates soft X-rays, which have a long wavelength and are useful for imaging nanoscale objects. Hajdu had theorized that a single diffraction pattern also could be obtained from an atomic-scale object—such as a macromolecule, virus or cell—using an ultra-short and extremely bright X-ray free-electron laser pulse before the sample explodes and turns into a plasma.

Now that the workability of this theory has been demonstrated with soft X-rays, scientists predict that they also will be able to apply the technique using "hard X-rays," which have a much shorter wavelength suitable for studying much smaller objects, such as the atomic-scale experiments theorized by Hajdu. The LCLS at SLAC will be the first free-electron laser capable of producing hard X-rays, providing an ideal platform for studying atomic- and molecule-sized objects, such as proteins.

Current techniques for imaging biomolecules with X-rays require that samples be grown into relatively large crystals the size of a grain of salt that contain numerous molecules arranged in a regular pattern. But many biomolecules resist crystallization. Because the flash technique can image individual molecules, scientists say that it will finally enable them to closely and rapidly study all classes of proteins.

"The entire collaboration is very excited by these results," said Hajdu. "Flash imaging has implications for studying molecular structures in biology in a whole new way. A new scientific community is forming to achieve these goals by combining biology with atomic, plasma and astrophysics for the first time."

Scientists from LLNL, Uppsala University, SLAC, DESY, Technische Universität Berlin, the Center for Biophotonics Science and Technology at the University of California-Davis and the private firm Spiller X-ray Optics of Livermore participated in the experiment.

The work was funded in part by the U.S. Department of Energy Office of Science, a Laboratory Directed Research and Development strategic initiative proposal for "biological imaging with fourth-generation light sources" at LLNL and the Swedish Research Councils.

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