Physicists move out electrons to help with identification

Using short, intense X-ray pulses from a free-electron laser (one with a beam of electrons accelerated to almost light speed) a team of physicists led by Western Michigan University’s Dr. Nora Berrah are providing a new way to explore atomic structure.

Work at the Stanford Linear Accelerator Center at Stanford University in Menlo Park, Calif. has resulted in direct observations of what has previously been a theoretical concept regarding the ability to tell the difference between similar chemical systems.

The international team of physicists led by Berrah used the free-electron laser pulse to displace the two innermost electrons from each of the atoms that form the molecule they were studying, creating a double-core hole. This effect needs to be achieved very fast -- within a few femtoseconds, or quadrillionths of a second.

The key innovation the team developed was demonstrating that double-core holes can be measured to fingerprint molecules that are so similar their spectral lines can’t be resolved using other spectroscopic techniques that use single-core-hole ionization.

An article on the work reviewed by appeared in an October issue of the Proceedings of the National Academy of Sciences. It is the first published piece that compares the work and earlier theoretical modeling on how molecules can be differentiated by using free-electron lasers to create a double-core hole by ejecting two electrons from their positions.

"Plotting our experimental data against the theory demonstrates clearly the agreement, which is very good concerning the structures we observed," Berrah says in the article. "However, there are discrepancies regarding the intensity of the structures, which may be due to effects not included in the calculations. That being said, the calculations guided us very well."

Berrah is an atomic physicist and longtime researcher whose work has been regularly funded by the U.S. Department of Energy. She is on sabbatical from the university this year and is in France doing research at the light source facility in Paris called SOLEIL.

Writer: Kathy Jennings, Second Wave
Source: Cheryl Roland, Western Michigan University
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