X-ray laser creates hollow atoms

08 Jul 2010

Researchers publish the first results of how intense X-rays interact with both atoms and molecules.

Creating hollow atoms

The first published scientific results from the world's most powerful hard X-ray laser, located at the Department of Energy's SLAC National Accelerator Laboratory, California, US, show its unique ability to control the behaviours of individual electrons within simple atoms and molecules by stripping them away one by one, in some cases creating hollow atoms (Nature 466 56) (Phys. Rev. Lett. 104 253002).

These early results describe in great detail how the Linac Coherent Light Source's (LCLS) intense pulses of X-ray light change the atoms and molecules that they are designed to image. Controlling those changes will be critical to achieving the atomic-scale images of biological molecules and movies of chemical processes that the LCLS is designed to produce.

In the Nature paper, a team led by Argonne National Laboratory physicist Linda Young describes how it tuned the LCLS pulses to selectively strip electrons, one by one, from atoms of neon gas. By varying the photon energies of the pulses, the researchers could do this from the outside in or – a more difficult task – from the inside out, creating so-called hollow atoms.

Young, who led the first experiments with collaborators from SLAC and five other institutions, said: "No-one has ever had access to X-rays of this intensity, so the way in which ultra-intense X-rays interact with matter was completely unknown. It was important to establish these basic interaction mechanisms."

Although previous experiments with intense optical lasers had stripped neon atoms of most of their electrons, Young's was the first to discover how ultra-intense X-ray lasers do this. At low photon energies, the outer electrons are removed, leaving the inner electrons untouched. However, at higher photon energies, the inner electrons are the first to be ejected; then the outer electrons cascade into the empty inner core, only to be kicked out by later parts of the same X-ray pulse.

According to Young, even within the span of a single pulse there may be times when both inner electrons are missing, creating a hollow atom that is transparent to X-rays.

"The transparency associated with hollow atoms could be a useful property for future imaging experiments, because it decreases the fraction of photons doing damage and allows a higher percentage of photons to scatter off the atom and create the image," she explained.

In the Physical Review Letters report, a team led by physicist Nora Berrah of Western Michigan University describes the first experiments on molecules. Her group also created hollow atoms, in this case within molecules of nitrogen gas, and found surprising differences in the way short and long laser pulses of exactly the same energies stripped and damaged the nitrogen molecules.

"We introduced molecules into the chamber and looked at what was coming out there, and we found surprising new science," said Matthias Hoener, a postdoctoral researcher in Berrah's group. "Now we know that by reducing the pulse length, the interaction with the molecule becomes less violent."

Berrah's team bombarded nitrogen gas with laser pulses that ranged in duration from about 4 to 280 fs. No matter how short or long it was, though, each pulse contained the same amount of energy in the form of X-ray light.

To the group's surprise, it found that the longer pulses stripped every single electron from the nitrogen molecules, starting with the ones closest to the nucleus; whilst the shorter pulses stripped off only some of them.

Their report attributes this to the frustrated absorption effect. Since the molecule's electrons are preferentially stripped from the innermost shells, there is simply not enough time during a short pulse for the molecule's outermost electrons to refill the innermost shells and get kicked out in turn.

Further experiments have investigated nanoclusters of atoms, protein nanocrystals and even individual viruses, with results expected to be published in coming months.