November 12, 2012
Supercharged Atoms Force Rethink On Existing Theories
Brett Smith for redOrbit.com - Your Universe Online
An international team of researchers has created ionized xenon atoms so positively charged that scientists involved in the study are now saying the existing theories need to be reexamined.Using the world´s most powerful laser, the Linac Coherent Light Source (LCLS) at the U.S. Department of Energy's (DOE) SLAC National Accelerator Laboratory at Stanford University, the physicists were able to kick out up to 36 electrons from a xenon atom in a single burst by tuning the laser to a specific resonance, 1.5 kiloelectronvolts (1.5 keV).
"Our results give a 'recipe' for maximizing the loss of electrons in a sample," Daniel Rolles, a researcher for the Max Planck Advanced Study Group at the Center for Free-Electron Laser Science (CFELS) in Hamburg, Germany, who led the experiments, said in a statement. "For instance, researchers can use our findings if they're interested in creating a very highly charged plasma. Or, if the supercharged state isn't part of the study, they can use our findings to know what X-ray energies to avoid."
According to the team´s report in Nature Photonics, the resonance, or energy range, of the laser caused atoms and molecules to resonate and eject electrons at rates that normally require higher energies. The 1.5 keV resonance was found to produce optimum results–better results than even the higher energy 2.0 keV pulse.
Previous experiments have shown that resonance can be used to achieve the ejection of electrons, but these experiments were not performed on atoms as heavy as xenon or using a laser as powerful as the LCLS.
"It was the highest charge state ever observed with a single X-ray pulse, which shows that the existing theoretical approaches have to be modified,” Rolles said.
When photons from the laser strike the atom some electrons are ejected and others enter an excited state. As these excited electrons return to their initial state, they release energy which can push another nearby excited electron out of the atom. Excited electrons can also be struck by another photon–ejecting it from the atom.
Despite the fact that the powerful X-rays of LCLS quickly destroyed the xenon samples being studied, the scientists´ ability to delay damage by microseconds proved critical in producing data and images.
Rolles said that identifying the resonance region for specific elements can be important when exposing them to different types of radiation so that any potential damage can be minimized.
"Most biological samples have some heavy atoms embedded," he explained.
The team at the SLAC National Accelerator Laboratory was able to perform the groundbreaking research using an experimental station built by fellow scientists at the Max Planck Advanced Study Group in Germany. The equipment, which weighed about 11 tons, was shipped to California in 40 crates. It has been at LCLS for three years and was used in more than 20 experiments.
"Reassembling this machine at LCLS within one month and then commissioning it and doing a science experiment in only seven days was an absolutely incredible feat," said Rolles.
After performing the research on xenon, the researchers have since done similar experiments with krypton and other heavy atoms. These experiments were led by Artem Rudenko of Kansas State University.