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Watching Ultrafast Bond
|To time-stamp the arrival of each x-ray pulse, researchers use an electro-optic crystal (green) placed next to the electron beam (white) in the linear accelerator just before the beam produces x-rays. A laser (red) probes changes in the crystal to measure the exact time the beam passed by. This image was created by Jean Charles Castagna, SLAC.
Scientists from DOE's Stanford Linear Accelerator Center (SLAC), Brookhaven National Laboratory, Argonne National Laboratory, Lawrence Berkeley National Laboratory, and Lawrence Livermore National Laboratory, along with other institutions, used advanced x-ray tools to see the first instants of change in a solid brought to the edge of melting. Their results were published in the February 2, 2007 issue of Science. At SLAC's Sub-Picosecond Pulse Source (SPPS), researchers used x-ray pulses just quadrillionths of a second long to study atomic interactions in the semi-metal bismuth.
When bismuth is struck with a laser, its electrons are excited, but not enough to actually melt the material. The x-ray pulses in the SPPS experiment allow researchers to look at the less-than-nanosecond period between the moment of the laser's expulsion to the time when the electrons return to their normal state.
The laser instantly changes bismuth's potential energy surface — a measure of the forces that hold solids together — thus weakening bismuth's atomic bonds. This puts the atom's nucleus out of equilibrium, like moving a marble from the center of a bowl up its sloped sides. To get back into equilibrium, the atom “rolls” to the center of the bowl and oscillates around the lowest point, like a marble before it comes to rest in the center of the bowl. The researchers measured the frequency of these oscillations to determine the forces that bind the atoms together. This is the first time-dependent mapping of a solid's potential energy surface. The results also back a theoretical framework used to predict potential energy surfaces for systems that are in equilibrium.
Although the SPPS has since been dismantled to make way for the Linear Coherent Light Source at SLAC, learning how to construct and operate more complex systems based on this technique could lead to advances in medicine, alternative energy, and other fields.
Submitted by DOE's Brookhaven National Laboratory
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