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When light shines on an atom, it can knock electrons loose. Scientists have studied this process for decades, but many questions remain, especially when atoms absorb multiple particles of light at the same time.

A team of international researchers have been using high-performance computing resources from the U.S. National Science Foundation's ACCESS program, including the Expanse system at the University of California San Diego Halıcıoğlu School of Data Science and Computing San Diego Supercomputer Center (SDSC), Bridges-2 at the Pittsburgh Supercomputing Center (PSC) and Stampede3 at the Texas Advanced Computing Center (TACC), for several years to investigate how electrons behave when exposed to different types of laser light.

“We used a combination of powerful lasers, precise experiments and supercomputer simulations to better understand how the direction of the light’s electric field affects the way electrons are ejected from atoms,” said Klaus Bartschat, a physics professor at Drake University in Des Moines (Iowa). “This is part of a much larger effort to understand how light and matter interact at the quantum level.”

The team's findings, which are based on work being conducted since 2019, were recently published in Physical Review A, a journal of the American Physical Society. Their research provides new insights into the behavior of electrons and could eventually contribute to advances in imaging technologies, new materials and laser-based applications.

Watching Electrons Move

The experiments were performed at FERMI, a free-electron laser (FEL) facility in Trieste, Italy. Researchers directed specially prepared laser light at helium atoms, one of the simplest elements in nature.

The extreme ultraviolet light from the FEL was “circularly polarized,” meaning its electric field rotated like a spinning corkscrew as it traveled. The intensity was high enough to kick out one of the helium electrons and the frequency tuned such that the second electron was moved into an excited state of the residual ion, which was “oriented” due to the circular polarization. Scientists then tested two different rotation directions, counter- and co-rotating, relative to circularly-polarized light from a strong infrared (IR) laser and observed how the remaining electron responded.

They discovered that changing the direction of the relative rotation altered the way electrons were emitted from the helium ions. Furthermore, the intensity of the laser light also affected the electrons' speed and emission direction. By measuring these patterns, the researchers were able to observe how multiple light particles, or photons from the IR laser, interacted with the same atom.

This is important because, when atoms absorb several photons at once, many possible quantum pathways are available simultaneously. These pathways can interfere with one another, making the process far more complicated than when an atom absorbs just a single photon.

The Role of Supercomputers

To help explain what they observed in the laboratory, the researchers ran detailed computer simulations using NSF ACCESS allocations on SDSC’s Expanse, PSC’s Bridges-2 and TACC’s Stampede3 systems, as well as TACC’s Frontera.

“The NSF ACCESS allocations allowed us to run simulations that predicted how electrons should behave under the experimental conditions,” Bartschat said. “When we compared the predictions with the actual measurements, the results matched remarkably well.”

The close agreement confirmed that the researchers' quantum-mechanical models accurately described what was happening inside the helium ions. Since this was somewhat expected due to the simplicity of this particular system, it also provided confidence in the experimental study.  Most importantly, the calculations helped calibrate critical laser parameters, such as the temporal intensity profiles.  Knowing these parameters accurately makes it possible to perform reliable benchmark experiments on systems that are currently too complex for theory to handle.

Why It Matters

Understanding how electrons respond to light is a fundamental goal of quantum physics. The better scientists understand these interactions, the better they can control and measure processes that occur on extremely small scales and incredibly short timeframes.

Researchers ultimately hope to achieve what is known as a “complete experiment,” one that captures all the information needed to fully describe a quantum process. While that goal remains out of reach for all but the simplest systems, studies like this bring scientists closer by revealing how different quantum pathways interact and influence one another.

“This experiment showed that we can control electron emission patterns by changing the direction of polarized light and adjusting the laser intensity,” Bartschat said. “That gives us a powerful new tool for studying more complex systems in the future.”

The work represents another step toward a deeper understanding of the quantum world and may help enable future advances in precision measurement, quantum technologies and ultrafast science.

The computational work was supported by the NSF ACCESS (allocation no. MCA08X034).