"Much like a sports photographer must use a camera with a very fast shutter speed to capture an athlete in motion without blurring, to be able to watch electrons moving, we needed to use a exceedingly short and very bright (powerful) pulse of X-rays," says Jordan Katz, the lead author on the Science paper who is now with Denison University. "For this study, the X-rays were produced at Argonne National Laboratory's Advanced Photon Source."
In addition to short bright pulses of X-rays, Katz said he and his co-authors also had to design an experimental system in which they could turn on desired reactions with an ultrafast switch.
"The only way to do that on the necessary timescale is with light, in this case an ultrafast laser," Katz says. "What we needed was a system in which the electron we wanted to study could be immediately injected into the iron oxide in response to absorption of light. This allowed us to effectively synchronize the transfer of many electrons into the iron oxide particles so that we could monitor their aggregate behavior."
With their time-resolved pump-probe spectroscopy system in combination with ab initio calculations performed by co-author Kevin Rosso of the Pacific Northwest National Laboratory, Gilbert, Katz and their colleagues determined that the rates at which electrons hop from one iron atom to the next in an iron oxide varies from a single hop per nanosecond to five hops per nanosecond, depending on the structure of the iron oxide. Their observations were consistent with the established model for describing electron behavior in materials such as iron oxides. In this model, electrons introduced into an iron oxide couple with phonons (vibrations of the atoms in a crystal lattice) to distort the lattice structure and create small energy wells or divots known as polarons.
"These electron small polarons effectively form a localized lower-valence metal site, and conduction
|Contact: Lynn Yarris|
DOE/Lawrence Berkeley National Laboratory