Physcial Sciences Division
Research Highlights

September 2008

An Electron's Exit

Scientists can now see the path electrons take when leaving exotic ions

For the first time, researchers tracked the path of electrons leaving multiply charged anions, molecules with a negative charge or electron at each end. When the molecule is short (top), the exiting electron takes a path nearly parallel to the molecule. On longer molecules (bottom), the electron takes a perpendicular path. Enlarged View

Results: Like a teen moving out on their own, an electron can go in a lot of different directions when it leaves its home molecule. Now, researchers can see the path the exiting electron takes thanks to scientists at Pacific Northwest National Laboratory and Washington State University.

With a combination of instruments, the team produced linear molecules with an added electron or negative charge at each end. They found that the electron's exit from the molecule was influenced by the nearness of the electron at the other end. If the other electron is close, just 3 atoms away, the electron beats a hasty retreat. If the negative charge is farther away, say 8 to 11 atoms, the exiting electron takes a more leisurely path leaving.

Why It Matters: This research established a new method for viewing the electron behavior of exotic anions and could shape future studies. Other scientists can use this instrument, available at the Department of Energy's EMSL, a national scientific user facility at PNNL, to ask new questions about molecules relevant to everything from energy storage to environmental remediation.

Methods: The team began with a simple hydrocarbon molecule with an electron or negative charge at each end. This molecule is called an ion, or more specifically a multiply charged anion. The team varied the number of carbon atoms between the two negative ends from 3 to 11. Then, they electrosprayed the ions into a photoelectron spectrometer.

Next, they fired a laser into the cloud of ions. The energy from the laser caused an electron to become excited and leave the molecule. The team captured the position of the electron using a phosphor screen and a CCD camera. With the images, they did a series of calculations and analyses to determine the angles of the electrons when they leave.

"This is the first photoelectron imaging of ions with multiple negative charges," said Dr. Lai-Sheng Wang, the principal investigator on the study, "these kinds of ions are quite common in nature, and now we have a new technique to study them.  That's exciting."

What's Next? The team plans to use this technique to examine other exotic molecules, such as sulfates that are linked to air pollution and carbonates for environmental remediation.

Acknowledgments: The U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Science Division and the National Science Foundation funded this research. The work was done by Xiao-Peng Xing and Xue-Bin Wang of Washington State University, and Lai-Sheng Wang, an Affiliate Senior Chief Scientist at PNNL and a professor at WSU.

This work was done at DOE's EMSL, a national scientific user facility at PNNL.

The work supports PNNL's mission to strengthen U.S. scientific foundations for innovation by developing tools and understanding required to control chemical and physical processes in complex multiphase environments.

Reference: Xing XP, XB Wang, and LS Wang. 2008. "Imaging Intramolecular Coulomb Repulsions in Multiply Charged Anions." Physical Review Letters 101:083003.