Defying expectations, x-ray laser supercharges atoms
MENLO PARK, Calif. – Using a single flash from the world’s most powerful x-ray laser, researchers have stripped
a record number of electrons from xenon atoms, creating a “supercharged,” strongly positive state at energies previously thought too low.
The findings defy theoretical calculations suggesting that up to 26 of the 54 electrons of the noble gas could be kicked out at the energy used; the remaining electrons are too strongly bound. However, the scientists discovered that up to 36 electrons flew from the atoms.
“To our knowledge, this is the highest ionization that has ever been achieved in an atom using a single electromagnetic pulse,” said experiment leader Daniel Rolles, a researcher for the Max Planck Advanced Study Group (ASG) at the Center for Free-Electron Laser Science (CFEL) in Hamburg, Germany. “Our observation shows that the existing theoretical approaches have to be modified.”
The ultrabright x-ray laser pulses of the LCLS at SLAC National Accelerator Laboratory can be used to strip electrons away from atoms, creating ions with strong charges. The ability to interact with atoms is critical for making the highest-resolution images of molecules and movies of chemical processes.
When an atom loses electrons, it acquires a positive electric charge and becomes ionized. As more electrons are torn from the atom, its ionization becomes stronger.
The researchers from 19 research centers around the world fired intense x-ray laser flashes from the Linac Coherent Light Source (LCLS) at the US Department of Energy’s SLAC National Accelerator Laboratory at atoms of xenon. With 1.5 keV, the photons of the x-ray radiation had about 1000 times more energy than visible light. When such a high-energy photon hits an electron in the xenon atomic shell, its energy is transferred to the electron. Through this collision, the electron can be ejected from the atomic shell – depending on how strongly it is bound.
Just as a stretched guitar string can vibrate and sustain a note, a specific tuning of the laser’s properties can cause atoms and molecules to resonate. The resonance excites the atoms and causes them to shake off electrons at a rate that otherwise would require higher energies.
“The LCLS experiment produced an unexpected and unprecedented charge state by ejecting dozens of electrons from an atom,” said graduate student and co-author Benedikt Rudek of the Max Planck ASG and the Max Planck Institute for Nuclear Physics. “The absorbed energy per atom was more than twice as high as expected.”
At an energy of 1.5 keV, this resonance effect was particularly strong for xenon. Consequently, even at a higher energy of 2 keV, the researchers observed only less strongly ionized atoms. Based on the measurements, the CFEL scientists refined a computational model that allows them to calculate such resonances in heavy atoms.
“Our results give a ‘recipe’ for maximizing the loss of electrons in a sample,” Rolles said. “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.”
Specialized equipment known as the CAMP chamber, pictured here, played a key role in advanced research at SLAC’s free-electron laser, the LCLS. A new paper details experiments with CAMP that observed a record supercharged state in xenon atoms. The equipment was on loan to SLAC through a collaboration with the Max Planck Society Advanced Study Group.
When investigating biological samples, however, most scientists should avoid the resonance regions of such heavy atoms because these regions can damage samples, affecting image quality, Rolles said.
In subsequent experiments, the investigators used the LCLS to examine krypton and molecules that contain other heavy atoms, said Artem Rudenko of Kansas State University, who led a recent follow-up experiment.
The precision measurements were conducted using a special experimental station, the CFEL-ASG Multi-Purpose (CAMP) chamber, which was shipped to SLAC for the three-year project. It was used in more than 20 experiments ranging from atomic and molecular physics to material sciences and bioimaging.
The research was reported in Nature Photonics (doi: 10.1038/nphoton.2012.261).
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