A team of researchers at the Department of Energy’s SLAC National Accelerator Laboratory has gained new insights into the photoelectric effect, a phenomenon first described by Einstein over a century ago. Their method provides a new tool for studying electron-electron interactions that are fundamental to many technologies, including semiconductors and solar cells.

The results were published on August 21 in Nature.

When an atom or molecule absorbs a photon of light, it can emit an electron in a process known as the photoelectric effect.

Einstein’s description of the photoelectric effect, also called photoionization, laid the theoretical foundation for quantum mechanics. However, the instantaneous nature of this effect has been the subject of intense study and debate.

Recent advances in attosecond research have provided the necessary tools to address the ultrafast time delays in photoionization.

“Einstein won the Nobel Prize for describing the photoelectric effect, but a hundred years later we have only just begun to truly understand the underlying dynamics,” said lead author and SLAC scientist Taran Driver.

“Our work represents a significant advance because we measure these delays in the X-ray range, a feat that has not been achieved before.”

The team used an attosecond X-ray pulse from SLAC’s Linac Coherent Light Source (LCLS), just billionths of a billionth of a second long, to ionize electrons at the nuclear level. This process released the electrons from the molecules they were studying.

They then used a separate laser pulse that sent the electrons in a slightly different direction depending on when they were emitted to measure what is known as the “photoemission delay.”

The photoemission delay can be thought of as the time between the absorption of a photon by a molecule and the emission of an electron. These delays, which are up to 700 attoseconds, are significantly larger than previously predicted. They challenge existing theoretical models and open up new ways to understand electron behavior. The researchers also discovered that interactions between electrons play an important role in this delay.

“By measuring the angular difference in the direction of the ejected electrons, we were able to determine the time delay with high precision,” said co-author and SLAC scientist James Cryan.

“The ability to measure and interpret these delays helps scientists better analyze experimental results, especially in areas such as protein crystallography and medical imaging, where the interaction of X-rays with matter is critical.”

The study is one of the first in a series of planned experiments aimed at exploring the depths of electron dynamics in various molecular systems. Other research groups have already begun to use the developed technique to study larger and more complex molecules, uncovering new facets of electron behavior and molecular structure.

“This is an evolving field,” said co-author Agostino Marinelli. “The flexibility of LCLS allows us to probe a wide range of energies and molecular systems, making it a powerful tool for these types of measurements. This is just the beginning of what we can achieve at these extreme timescales.”