UW-Madison engineers helped reveal the answer to a quantum-mechanical mystery based on measurements of the behavior of electrons on unprecedentedly small time-scales.
The results, published online June 2, 2016, in the journal Science, came from collaboration between researchers at UW-Madison and a team at the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, who developed a powerful new device for monitoring subatomic particles. Data from the instrument allowed Tibor Szilvási, a postdoctoral researcher in chemical and biological engineering under Vilas Distinguished Achievement Professor and Paul A. Elfers Professor Manos Mavrikakis, to assist in explaining a longstanding puzzle in interactions between light and matter.
Materials can give off electrons after absorbing incoming high-energy photons. Fundamental physical laws and common sense predict that energy should transfer instantaneously, yet researchers sometimes observe a tiny time lag between the moments that incoming beams strike some surfaces and when electrons start flowing. What exactly the elementary particles could be doing to cause this delay long defied explanation, partly because tools didn’t exist to accurately measure such infinitesimal spans of time.
Generating electrons requires high-power X-ray sources. Previously, the only facilities that could make such measurements were specialized large-scale particle accelerators, requiring mile after mile of underground tunnels and precision equipment. Scientists and engineers at NIST and the University of Colorado developed a device with the same capabilities at a fraction of the size—small enough, in fact, to fit on top of a typical lab bench. This tabletop energy source, combined with intense, finely tuned lasers, enabled the scientists to observe particles one quintillionth of a second at a time.
Unprecedented, attosecond-scale measurements in combination with theoretical calculations revealed that electrons don’t leave materials immediately after incoming light strikes the surface. Rather, the tiny charged particles transition to an intermediate energetic state before starting to flow freely, explaining the tiny time delay.
“The new device pushes the limits of experimental techniques,” says Szilvási, who is among the the paper’s coauthors from UW-Madison. “It generates X-rays at a fraction of the cost as traditional sources.”
The researchers plan to characterize more materials using this new instrument. The National Science Foundation, the Department of Energy, and the Air Force Office of Scientific Research supported the project.
Author: Sam Million-Weaver