In experiments at SLAC National Accelerator
Laboratory, a powerful X-ray laser blasted solid carbon crystals into a liquid
and plasma even faster than expected, raising new questions about how these
intense beams interact with matter.
The tests took place at the Linac Coherent
Light Source, or LCLS, using a pioneering technique to simultaneously blast and
probe samples of graphite, a pure form of carbon.
The team chose graphite partly because it
might offer a way to see whether biological molecules, which are also
carbon-based, will produce useful data when probed with intense X-ray laser
pulses, said Stefan P. Hau-Riege, a staff physicist at Lawrence Livermore
National Laboratory who led the research team. In addition, its fundamental
properties, such as its melting behavior, are still not well understood.
Hau-Riege said the results, which show
ultrafast changes from solid to liquid and from solid to plasma in the
graphite, defied the team’s expectations. “The models that we’re currently
using don’t explain it,” he said. There are “processes taking place that we
don’t fully understand.”
He added that these results could have
implications for a range of future experiments using X-ray free-electron
lasers, or XFELs.
In a paper scheduled for publication in Physical Review Letters, the
researchers note that the type of change they saw in the graphite samples could
be an obstacle to using free-electron lasers to image single particles and
crystals with atomic resolution, “since X-ray damage proceeds faster
than anticipated.”
The team included Marc Messerschmidt,
Christoph Bostedt and Sebastian Schorb, who work at SLAC’s LCLS facility. Other
collaborators were from the University of Duisburg-Essen, the Max Planck
Advanced Study Group’s Center for Free Electron Laser Science, the Max Planck
Institut für Medizinische Forschung, and the Max Planck Institut für Kernphysik
in Germany.
Researchers commonly employ XFELs for
“probe before destroy” experiments: They hit the sample with an X-ray
laser pulse and extract as much useful data as possible before the sample is
damaged.
The planes of a crystal sample can function
like tiny mirrors, and the Livermore-led team used the Atomic, Molecular and
Optical instrument at LCLS to observe this reflectivity, gathering data from
the scattering of X-rays as they hit the graphite sample with laser pulses just
40 to 80 femtoseconds (quadrillionths of a second) long.
Researchers found that, based on their
results, the models they used to simulate the stressed properties of graphite
significantly underestimated the extent and timing of the material’s
transformation when struck by the X-ray laser.
Hau-Riege said he is planning follow-up
research with LCLS’ Coherent X-ray Imaging instrument in July to study silicon
samples.
Both studies are part of ongoing research
to study materials’ reaction to high-energy XFEL pulses, and to identify
materials that are most resistant to damage in XFEL experiments.
Hau-Riege
said LCLS is an attractive facility for experiments because it allows researchers
to simultaneously study various states of matter using a variety of imaging
techniques. He has also been a part of similar experiments, including XFEL
tests of multilayered materials using the FLASH soft X-ray laser at the
Deutsches Elektronen-Synchrotron (DESY) in Germany.
