SLAC Researchers Capture First Sequence of Coster-Kronig Decay in Molecules

The discovery was made possible by timing control pioneered by SLAC accelerator scientists, which allowed researchers to adjust the delay between two X-ray flashes with attosecond accuracy. The first flash energized the molecule, while the second flash, arriving attoseconds later, provided insight into the movement of the electrons.

The Stages of Molecular Decay

The research team identified a distinct sequence of events occurring immediately after the molecule was energized:

• Less than 1 femtosecond: The molecule relaxed by ejecting an electron from one of its inner shells. This Coster-Kronig decay creates a low-energy electron that moves slowly enough to interact with surrounding molecules, a process that can break DNA strands and cause radiation damage in biological systems.
• 2 to 10 femtoseconds: The ejected electron left behind a hole that migrated through the molecule until another electron filled it. This motion was driven by quantum coherence.
• More than 10 femtoseconds: The electron movement resulted in chemical consequences, with new chemical bonds beginning to form and existing bonds breaking.

The Role of Quantum Coherence

The fleeting motion observed between two and 10 femtoseconds is attributed to electronic coherence. According to Driver, a member of the research team, there is a belief that this coherence may influence the downstream effects of the reaction, specifically the forming and breaking of chemical bonds. Driver stated, By capturing and characterizing electronic coherence, we can better understand, and perhaps one day gain some measure of control over its effects.

Refining Theoretical Models

The project revealed a gap between existing computer simulations and actual electron behavior. Alicia Palacios, an associate professor at the Autonomous University of Madrid, explained that computer models typically focus on specific types of electron motion while keeping atomic nuclei fixed in place because modeling ultrafast molecular motion is highly complex.

Palacios noted that while incorporating additional complexity makes calculations more computationally demanding, they produce simulations that are much closer to the reality captured during experiments. The team utilized these adjusted theoretical models alongside experimental evidence to more reliably predict the outcomes of photochemical reactions.

Future Research and Support

The data for this study was collected in 2021, a process that took more than a week to gather the information necessary for comparison with predictions. The team is currently developing next-generation experiments designed to collect more data in less time, with the goal of mapping electron movements in larger and more complex molecules.

This research was supported in part by the DOE Office of Science, and the LCLS served as the DOE Office of Science user facility for the study.