Unpeeling Atoms and Molecules from the Inside OutNo longer in the vanguard of particle physics, the Stanford Linear Accelerator Center (SLAC) has converted its Nobel-winning particle smasher into a powerful electron-based x-ray laser, the Linac Coherent Light Source (LCLS). As the first scientific results from LCLS are published, Kenneth Chang of the New York Times looks at how LCLS has transformed and rejuvenated SLAC.
First Results from the Linac Coherent Light Source (LCLS)
Stanford Linear Accelerator Center | 30 June 2010
Menlo Park, Calif.—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, show its unique ability to control the behaviors of individual electrons within simple atoms and molecules by stripping them away, one by one—in some cases creating hollow atoms.
The world's first hard X-ray free-electron laser started
operation with a bang. First experiments at SLAC
National Accelerator Laboratory's Linac Coherent Light
Source stripped electrons one by one from neon atoms
(illustrated above) and nitrogen molecules, in some
cases removing only the innermost electrons to create
"hollow atoms." Understanding how the machine's ultra-
bright X-ray pulses interact with matter will be critical
for making clear, atomic-scale images of biological
molecules and movies of chemical processes.
(Artwork by Gregory Stewart, SLAC.)
These early results—one published today, the other last week—describe in great detail how the Linac Coherent Light Source's intense pulses of X-ray light change the very atoms and molecules 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 a report published in the July 1 issue of Nature1, a team led by Argonne National Laboratory physicist Linda Young describes how they were able to tune LCLS pulses to selectively strip electrons, one by one, from atoms of neon gas. By varying the photon energies of the pulses, they could do it from the outside in or—a more difficult task—from the inside out, creating so-called "hollow atoms."
...
In another report, published June 22 in Physical Review Letters2, a team led by physicist Nora Berrah of Western Michigan University—the third group to conduct experiments at the LCLS—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.
- Femtosecond electronic response of atoms to ultra-intense X-rays
- Nature 466 (01 July 2010) DOI: 10.1038/nature09177
- Ultraintense X-Ray Induced Ionization, Dissociation, and Frustrated Absorption in Molecular Nitrogen
- Physical Review Letters 104 253002 (23 June 2010) DOI: 10.1103/PhysRevLett.104.253002
American Physical Society | Physics Synopsis | 23 June 2010
Ultraintense X-Ray Induced Ionization, Dissociation, and Frustrated Absorption in Molecular Nitrogen
Imaging the internal dynamics of excited atoms and molecules with high spatial resolution requires fast, energetic pulses of x rays. As Matthias Hoener and colleagues from an international collaboration report in Physical Review Letters, initial results on how molecules absorb high-energy, femtosecond x-ray pulses are now emerging from the Linac Coherent Light Source (LCLS), which began operation last year at SLAC National Accelerator Laboratory in California, US.
Hoener et al. used the LCLS free-electron laser to produce x-ray pulses with wavelength 1.1 nm (1100 eV photon energy) and pulse widths from 280 femtoseconds down to 4 femtoseconds. The team directed the pulses at puffs of nitrogen (N2) gas and analyzed the resulting ionized molecules with a time-of-flight mass spectrometer.
All the x-ray pulses delivered the same total amount of energy to the nitrogen gas, but Hoener et al. observed that the spectrum of ions depended on the pulse width: Long duration pulses completely stripped the electrons from the nitrogen atoms, but shorter pulses were not able to produce the fully ionized state. Hoener et al. argue that for shorter pulses, the cycle of an outer-shell electron falling into the emptied core-shell (Auger decay) cannot occur quickly enough, limiting the achievable ionization.
The experiments reveal the fundamental dynamics of how molecules absorb high-intensity, hard x rays and bear directly on what may be possible in future single-shot studies of chemical structure and dynamics—one of the most important applications of free-electron lasers. – David Voss