The Science
Scientists used a free-electron laser to measure delays as short as an attosecond in X-ray photoemission. Photoemission is where an atom or molecule absorbs light and emits an electron. Although this is a fundamental process, scientists do not yet fully understand it. Using the rotating electric field of an infrared laser pulse as a stopwatch, the scientists measured this delay in nitric oxide molecules. The team observed a delay nearly twice as long as predicted. This result challenges existing theoretical models. The longer delay results from interactions between electrons during photoemission. Such interactions are foundational to technologies like semiconductors. However, they remain extremely difficult for scientists to model with high accuracy.
The Impact
With this measurement, scientists have gained a powerful new tool for investigating electron–electron interactions. This is a key step toward building more accurate models for quantum physics. In addition, understanding the timing of delays in X-ray photoemission makes it possible for scientists to interpret experimental data more precisely. This capability is especially valuable in fields where the interactions between matter and X-rays are central. These fields include protein crystallography, which uncovers molecular structures, and medical imaging, which enables advanced diagnostics. By helping scientists refine theoretical models, the findings advance both fundamental physics and practical applications of X-ray science.
Summary
When an atom or molecule absorbs a photon, it can release an electron. This process is known as photoemission, or the photoelectric effect. Einstein's explanation of this phenomenon was pivotal in the development of quantum mechanics. Theory predicts that the delay between photon absorption and electron emission is vanishingly small, on the order of attoseconds (10-¹8 seconds). Groundbreaking experiments have measured such delays with ultraviolet light. However, until recently, scientists could not measure an X-ray photoemission delay. X-ray pulses short enough for precise timing did not exist. X-ray photoionization is significant because it probes core-level electrons, which are particularly sensitive to electron–electron interactions. The team found that for these core-level electrons, the photoemission delay was nearly twice what was expected from theory, due to these interactions between electrons. This can provide a better understanding of electron dynamics in molecules.
Using the Linac Coherent Light Source (LCLS), a DOE Office of Science User Facility, scientists generated X-ray pulses only a few hundred attoseconds long. They used these pulses to achieve the first measurement of X-ray photoemission delay. Specifically, they used a technique called angular streaking. In this approach, the X-ray pulse is combined with a circularly polarized infrared laser pulse. The laser's rapidly rotating electric field acts as an ultrafast clock. It maps the timing of electron release to its detection angle. This breakthrough provides an unprecedented tool to refine models of electron–electron interactions. It also opens pathways to technological advances that range from next-generation catalysts to quantum computing.
Funding
This research was partially supported by the Chemical Sciences, Geosciences, and Biosciences Division and the Scientific User Facilities Division of the Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (DOE), as well as by the German Research Foundation and Institute for Basic Science of Korea. Experiments were performed using resources at the Linac Coherent Light Source, a DOE Office of Science User Facility.