Xenon warriors' success: Supercharged

Xenon supercharged

It is now possible to use a coherent light source to strip electrons from relatively inert xenon atoms at much lower energies than previously considered plausible. Using the Linac Coherent Light Source (LCLS) at the US Department of Energy's SLAC National Accelerator Laboratory, the researchers involved have shown that they can create a "supercharged" form of xenon.

The high-energy X-rays generated by the LCLS are usually so extreme as to destroy samples but a for a fleeting instant, but Daniel Rolles of the Max Planck Advanced Study Group at the Center for Free-Electron Laser Science in Hamburg, Germany and his team have turned this behaviour to their advantage, Rolles and colleagues have used these X-rays to defy expectations and bust theoretical constraints by inducing significant "damage" in xenon atoms to allow them to study the extreme states of matter they might exist. They report details of the of X-rayed xenon work in the journal Nature Photonics. 

Maximising electron losses

"Our results give a 'recipe' for maximizing the loss of electrons in a sample," team leader Rolles explains. "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." So, not only does the work reveal how extreme states of matter might be generated it also shows how they can be avoided if the aim is to study more sensitive systems. It is the laser light that, like the sustained notes of a Gibson Les Paul, can be tuned to cause resonance in the atoms or molecules being studied and so excite the system to shake off its electrons at lower energies than would normally be required.

Previously, the team had experimented in a laser facility in Germany with exposure of atoms to pulses of ultraviolet light but the higher-energy beams available at the LCLS have now allowed them to extend their studies much further. While it is common knowledge that triggering resonances in atoms will affect their charged states, "it was not clear to anybody what a dramatic effect this could have in heavy atoms when they are being ionized by a source like LCLS," Rolles explains. "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." 

Pushing the charged state

Team member Benedikt Rudek analysed the data resulting from the experiments and explains that, "The LCLS experiment pushed the charged state to an unprecedented and unexpected extreme, more than doubling the expected energy absorbed per atom and ejecting dozens of electrons."

The researchers suggest that the understanding gained from this resonance study can now also be used to carry out X-ray studies without destroying a sample too quickly by allowing researchers to avoid the kind of supercharged plasma states Rolles' team were able to observe. This could also be exploited to improve imaging experiments. "Most biological samples have some heavy atoms embedded, for instance," Rolles said, and in some experiments, avoiding the resonance trigger might prevent rapid damage to those atoms. Indeed, the team has now demonstrated proof of principle with the element krypton as well as molecular systems containing relatively heavy atoms.

"The goal of this research is to better understand the interaction of super-intense X-rays with atoms and molecules on a fundamental level and to gain knowledge that pretty much helps anybody who needs to understand this interaction better," Rolles told SpectroscopyNOW. "However, we also have biologists on the team with whom we are using Free-Electron Laser pulses to image proteins whose structures are not known to date. In view of these experiments, we are particularly interested in the mechanisms and the timescales on which radiation damage occurs on a microscopic level."

The research team has scientists from 19 research centres including other Max Planck institutes, PNSensor GmbH, Technical University of Berlin, Jülich Research Center, University of Hamburg and Physikalisch-Technische Bundesanstalt in Germany, SLAC and Western Michigan and Kansas State universities in the US, University of Pierre and Marie Curie and National Center for Scientific Research in France; and Kyoto and Tohoku universities in Japan.