US scientists document ultrafast electron motion driving chemical bond changes

Capturing Electron Motion

To visualize the process, a research team used two precisely timed X-ray flashes from the Linac Coherent Light Source (LCLS). The first flash provided a jolt of energy by dislodging an electron, while a second flash, arriving attoseconds later, utilized X-ray absorption spectroscopy to track where electrons had moved. By adjusting the delay between flashes with attosecond accuracy, the team documented 10 timestamps within the first 10 femtoseconds.

The resulting sequence revealed several distinct stages of the reaction:

• Less than 1 femtosecond: The molecule relaxed by ejecting an electron from an inner shell. This Coster-Kronig decay creates a low-energy electron capable of causing radiation damage in biological systems and breaking DNA strands.
• 2 to 10 femtoseconds: A hole left by the ejected electron migrated through the molecule until another electron filled it, a process driven by quantum coherence.
• More than 10 femtoseconds: The chemical consequences of the electron movement became visible as chemical bonds began breaking and new ones formed.

Experimental Approach and Theory

In a separate effort to track a valence electron, a team led by Stanford University PhD student Ian Gabalski, Professor Philip Bucksbaum of the Stanford PULSE Institute, and Nanna List of the University of Birmingham and KTH Royal Institute of Technology used time-resolved X-ray scattering. The team used an enclosure of high-density ammonia, which was excited with an ultraviolet laser, because the valence electrons in small, light molecules like ammonia outnumber core electrons, creating a signal strong enough to track.

The experimental data indicated that actual electron behavior was more complex than previous models suggested. Alicia Palacios, an associate professor at the Autonomous University of Madrid, noted that while incorporating additional complexity into calculations is more computationally demanding, it produces simulations closer to the reality captured in experiments.