November 15, 2012
Supercharged: Researchers discover technique to kick a record number of electrons out of an atom with an X-ray laser
Supercharging is a technique no longer confined to automotive enthusiasts.
Artem Rudenko, a new assistant professor of physics at Kansas State University and member of the James R. Macdonald Laboratory, was one of the principal investigators in an international physics collaboration that used the world's most powerful X-ray laser to supercharge an atom. By stripping a record 36 electrons from a xenon atom, researchers were able to bring the atom to a high positively charged state thought to be unachievable with X-ray energy.
The findings will help scientists create and study extreme new states of matter, such as highly charged plasma, by fine-tuning the laser's X-ray radiation wavelengths in resonance with atomic levels -- resulting in ultra-efficient electron removal.
Conversely, researchers can use the findings to tune the laser wavelength to avoid enhanced electron stripping. This will reduce damage caused by X-rays and help produce better quality images of nano-world objects.
"Taking single-shot, real-time images of viruses, proteins or even smaller objects is a long-standing dream that came close to reality with the advent of powerful X-ray laser like the Linac Coherent Light Source," Rudenko said. "The main problem, however, is that such a laser also inevitably destroys the sample in the process of acquiring an image, and reducing this destruction by any means is critical for producing high-quality images."
The study on supercharging was performed through a large international collaboration led by Daniel Rolles from Max Planck Advanced Study Group, or ASG, in Hamburg, Germany, along with Rudenko and Joachim Ullrich, now a president of the PTB, the German national metrology institute.
"We brought 11 tons of equipment funded by the German Max-Planck Society to LCLS, which is a unique 1.5 km-long X-ray laser operated by Stanford University for the U.S. Department of Energy, and involved scientists from 19 research centers all over the world," Rudenko said. "We also needed to come back one year after our first experiment and repeat the measurements to understand the results. From all that we knew about this process we expected to strip at most 26 electrons, and it immediately became clear that the existing theoretical approaches have to be modified."
For the second leg of experiments physicists chose even higher X-ray energy -- and, surprisingly, saw fewer electrons kicked out of the atom. The key was that even though the energy was higher, it was not in resonance.
"While it is known that resonances in atoms affect their charged states, it was unclear what a dramatic effect this could have in heavy atoms like xenon under ultra-intense X-rays," Rudenko said. "Besides ejecting dozens of electrons, this more than doubled the energy absorbed per atom compared to all expectations."
Follow-up experiments led by Rudenko discovered similar effects in krypton atoms and several molecules.
The results were analyzed by Benedict Rudek from ASG Hamburg and reported in Nature Photonics journal in the article, "Ultra-efficient ionization of heavy atoms by intense X-ray free-electron laser pulses," http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2012.261.html.
For more information on the Linac Coherent Light Source, or LCLS, and the instrument used for the project, go to https://portal.slac.stanford.edu/sites/lcls_public/Pages/Default.aspx and http://today.slac.stanford.edu/feature/2009/lcls-camp.asp.