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WMU physicists lead groundbreaking atomic research

July 2, 2010

KALAMAZOO--A team of Western Michigan University researchers is among the first to show results of new work being done at a Stanford University-based national laboratory, and their work is garnering international attention by giving the scientific community its first look at what the world's most powerful hard X-ray laser can do.

The first published scientific results from the world's first x-ray laser, located at the U.S. Department of Energy's SLAC National Accelerator Laboratory in Menlo Park, Calif., show the x-ray laser's unique ability to control the behaviors of individual electrons within simple atoms and molecules by stripping them away, one by one--in some cases creating hollow atoms.

These early results were outlined in papers published June 22 and July 1 and written, respectively by the WMU-led team and a team from Argonne National Laboratory. They describe in great detail how SLAC's Linac Coherent Light Source's intense pulses of X-ray light change the very atoms and molecules they are designed to image. Controlling those changes will be critical to achieving the atomic-scale images of biological molecules and movies of chemical processes that the LCLS is designed to produce.

WMU team publishes first results

The first report from any experiments carried out at the LCLS was published June 22 in Physical Review Letters. In the report, a team led by WMU's veteran physics researcher Nora Berrah describes the results of their experiments on molecules. Berrah's team, which includes WMU post-doctoral researchers Matthias Hoener, Li Fang and Brendan Murphy, was one of the first groups to conduct experiments at the LCLS.

Berrah's team created hollow atoms and molecules within molecules of nitrogen gas, and found surprising differences in the way short and long laser pulses of exactly the same energies stripped and damaged the nitrogen molecules.

"We just introduced molecules into the chamber and looked at what was coming out there, and we found surprising new science," said Hoener, who in addition to being a WMU postdoctoral researcher is also a visiting scientist at Lawrence Berkeley National Laboratory and was first author of the paper. "Now we know that by reducing the pulse length, the interaction with the molecule becomes less violent."

The work and the resulting publication, Berrah says, was the result of a large collaboration that included researchers from other national laboratories and universities who are on the author list. In particular, Oleg Kornilov and Oliver Gessner from Lawrence Berkeley contributed the final model to the data. Steve Pratt from Argonne, Steve Leone from Lawrence Berkeley-UC Berkeley and Phil Buckbaum and Markus Guehr from SLAC/Stanford University also contributed important insights to the interpretation of the results.

Second report is in Nature

A second report published in the July 1 issue of Nature, describes how Argonne National Laboratory physicist Linda Young and her team, which includes Berrah, Fang and Hoener, were able to tune LCLS pulses to selectively strip electrons, one by one, from atoms of neon gas. By varying the photon energies of the pulses, the group could do it from the outside in or--a more difficult task--from the inside out, creating hollow atoms.

"Until very recently, few believed that a free-electron X-ray laser was even possible in principle, let alone capable of being used with this precision," said William Brinkman, director of DOE's Office of Science. "That's what makes these results so exciting."

Young, who led the first experiments in October with collaborators from SLAC and five other institutions, said, "No one has ever had access to X-rays of this intensity, so the way in which ultra-intense X-rays interact with matter was completely unknown. It was important to establish these basic interaction mechanisms."

Once just a dream

SLAC's Joachim Stohr, director of the LCLS, described the discoveries being made as a dream becoming a reality. "When we thought of the first experiments with LCLS ten years ago, we envisioned that the LCLS beam may actually be powerful enough to create hollow atoms, but at that time it was only a dream."

While the first experiments were designed to see what the LCLS can do and how its ultra-fast, ultra-bright pulses interact with atoms and molecules, they also pave the way for more complex experiments to come. Its unique capabilities make the LCLS a powerful tool for research in a wide range of fields, including physics, chemistry, biology, materials and energy sciences.

The LCLS forms images by scattering X-ray light off an atom, molecule or larger sample of material. Yet when the LCLS X-rays are tightly focused by mirrors, each powerful laser pulse destroys any sample it hits. Since certain types of damage, like the melting of a solid, are not instantaneous and only develop with time, the trick is to minimize the damage during the pulse itself and record the X-ray snapshot with a camera before the sample disintegrates.

Both teams found that the shorter the laser pulse, the fewer electrons are stripped away from the atom or molecule and the less damage is done. And both delved into the detailed mechanisms behind that damage.

International team involved

Berrah, a professor of physics at WMU, leads a team that, in addition to the three WMU post-docs, includes 26 scientists comes from the United States, Germany and Finland. The team spent five days conducting experiments at the LCLS last October. Their goal was to see how the LCLS pulses interacted with simple molecules of nitrogen gas, which consist of two nitrogen atoms bound together.

Berrah's team bombarded puffs of nitrogen gas with laser pulses that ranged in duration from about four femtoseconds, or quadrillionths of a second, to 280 femtoseconds. No matter how short or long it was, though, each pulse contained the same amount of energy in the form of X-ray light; so researchers might expect that they would have roughly the same effects on the nitrogen molecules.

But to the team's surprise, that was not the case, Hoener said. The long pulses stripped every single electron from the nitrogen molecules, starting with the ones closest to the nucleus; while the short ones stripped off only some of them.

The team's report attributes this to the "frustrated absorption effect." Since the molecule's electrons are preferentially stripped from the innermost shells, there is simply not enough time during a short pulse for the molecule's outermost electrons to refill the innermost shells and get kicked out in turn.

With all this activity going on inside the atom, scientists have a new way to explore atomic structure and dynamics. Further experiments have investigated nanoclusters of atoms, protein nanocrystals and even individual viruses, with results expected to be published in coming months.

Berrah's research was supported by the DOE Office of Science. Young's research was primarily supported by the DOE Office of Science, with additional support from the Alexander von Humboldt Foundation for Hoener's fellowship.

Read more
SLAC writes about the research results
Synopsis of The Physical Review Letters paper

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Media contact: Cheryl Roland, (269) 387-8400, cheryl.roland@wmich.edu

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