Ultrafast phase changes observed in graphite

Graphite once again has been shown to have ground-breaking potential due to its ability to phase change from a liquid to a warm-dense plasma in just 40 fs.

Researchers using the Linac Coherent Light Source (LCLS) X-ray Free-Electron Laser (XFEL) at the SLAC National Accelerator Laboratory at Stanford University used various pulse lengths and spectra to heat a sample of graphite to the point where it would phase change from solid to liquid, and then to a warm-dense plasma. What is remarkable is that the graphite made these phase changes in about 40 fs.

Demonstration of ultrafast disintegration of matter by 2-keV Linac Coherent Light Source pulses: The team combined techniques commonly used in solid-state physics (Bragg reflection) with techniques from plasma physics (spectroscopy of diffusely scattered light) to characterize ultrafast heating in graphite. (Image: LLNL)

"We found that the heating and disintegration of the ion lattice occurs much faster than anticipated," said Stefan Hau-Riege of Lawrence Livermore National Laboratory of Livermore, Calif.

From this experiment, the researchers were able to determine the time dependence of plasma parameters in the graphite, for example the electron and ion temperatures and the ionization states. Understanding these characteristics is important for the development of new material synthesis techniques and for high-energy density science. Other fields affected by this discovery are single-molecule biological imaging and x-ray optics.

The findings are to be published in the May 21 issue of the journal Physical Review Letters.

Also working on this research at Livermore are Alexander Graf, Tilo Doppner, Rich London, Carsten Formann, Siegfried Glenzer, Matthias Frank and Joe Bradley. Collaborating institutions include the University of Duisburg-Essen; Max Planck Advanced Study Group, Center for Free Electron Laser Science; Max Planck Institute for Medical Research; and Max Planck Institute for Nuclear Physics.

For more information, visit: www.llnl.gov