• Number 317  |
• August 2, 2010

First LCLS results document single-shot imaging

In the first published scientific results from the Linac Coherent Light Source (LCLS), an international collaboration led by Nora Berrah and Matthias Hoener of Western Michigan University (WMU) tested the concept of single-shot imaging by blowing apart nitrogen molecules with shorter and shorter bursts of intense x-rays.

The idea of single-shot imaging is that a single, short-enough pulse of bright x-rays can generate enough information about a sample, perhaps a virus or a strand of DNA, to record the position of all its atoms before the energetic pulse blows the sample apart.

The challenge, says Oliver Gessner of the Ultrafast X-ray Science Laboratory at DOE's Berkeley Lab, who led the Berkeley contingent, is that “until now, no one has ever tested what ultrashort pulses of ultra-intense x-rays, delivering an enormous amount of power in a very short time, actually do to molecules.”

The experiment used the LCLS’s Atomic, Molecular and Optical Science instrument and blasted simple nitrogen molecules with pulses ranging from 280 to only four femtoseconds in duration (a femtosecond is a quadrillionth of a second). Since each pulse packed the same number of photons (particles of light), the peak power increased as the pulses grew shorter. Nevertheless, the shortest pulses did the least damage.

The research team worked with x-rays whose photons interact almost exclusively with an atom’s core electrons, those in orbitals nearest the nucleus. When a photon knocks an inner electron out of an atom, one of the outer electrons quickly falls into the hole, in a process called Auger decay that provides enough energy for a third electron to be ejected. If more photons in the same x-ray pulse hit the same atom, core electrons can be ejected over and over again, each followed by Auger decay.

It was no surprise to the researchers that a “long” pulse of x-rays (280 quadrillonths of a second long!), hitting a nitrogen atom with one photon after another, could strip it of all seven of its electrons, reducing it to a bare core. Such highly charged ions in a molecule cause repulsive forces strong enough to blow it apart.

The research team’s crucial finding was that a way to produce only nitrogen molecules with low charge is with shorter pulse lengths, even though their peak power is higher.
When pulses are seven femtoseconds or less in duration, photons move through the atom so close together that Auger decay often can’t fill up core holes fast enough for photons later in the pulse to photoionize them again. Unfilled holes mean fewer inner electron targets. The number of highly charged atoms falls off sharply.

The researchers named this phenomenon “frustrated absorption,” a molecular mechanism that protects the integrity of molecules by preventing their constituent atoms from being stripped of the outermost valence electrons that hold them together.

“Although the molecular system we studied is very simple,” Gessner remarks, “our result is a start at describing the quantitative damage an illuminating pulse is liable to do to your target.”

From these observations, the research team constructed a model that may allow future researchers at the LCLS and similar facilities to calculate what kind of image distortion to expect from pulses of different lengths. The bottom line would appear to be: the shorter the pulse the less the damage.

The collaboration’s results are reported in the June 25 issue of Physical Review Letters and is available online to subscribers. In addition to the SLAC National Accelerator Laboratory, home to the LCLS, DOE laboratories including Lawrence Berkeley, Lawrence Livermore, and Argonne participated in the research collaboration. Besides WMU, universities including the University of California at Berkeley, Ohio State, Louisiana State, the University of Turku, Finland, and the Max Planck Advanced Study Group in Hamburg were also represented.

Submitted by DOE's Lawrence Berkeley National Laboratory