How did they capture the full picture?

The team combined two complementary X-ray methods at the Linac Coherent Light Source (LCLS) at SLAC:

• Element-specific X-ray absorption spectroscopy on the chemRIXS instrument, which tracked how electrons moved between specific molecular sites
• Time-resolved X-ray scattering on the X-ray Correlation Spectroscopy (XCS) instrument, which tracked atomic rearrangement and solvent motion

PNNL theorists Niranjan Govind and Amity Andersen supplied time-dependent density functional theory and molecular dynamics simulations to interpret the X-ray signals. Researchers at the University of Geneva helped identify the best experimental conditions and timescales. First author Abdullah Kahraman worked on the project at SLAC while he was a PNNL postdoctoral associate.

Why a ruthenium-based molecule?

The team chose a well-studied metal complex that absorbs light and, in acidic conditions, captures a proton from its surroundings. Co-investigator Christopher Larsen, a senior lecturer at the University of Auckland in New Zealand, explained that the molecule avoids extra electronic or structural changes that would muddy the X-ray signals.

What did they actually see?

Local changes in electronic structure at specific molecular sites occurred at the same time as a broader rearrangement of the surrounding water network. When the X-ray data and simulations agreed closely, the team could draw conclusions about the coordinated motion.

What remains unanswered?

Biasin listed several open questions: whether electrons and protons move together or in sequence, at which molecular site, and how the water network facilitates the proton hop. Many questions persist about other PCET reactions, especially the precise timing and order of each step.

What are the limitations?

X-rays cannot see protons directly, because protons carry almost no electrons. Biasin noted that the team observes the local reorganization of electronic structure and the broader reorganization of water, drawing inferences only where the two match.

The molecule under study is a model whose core mechanism was already known. More elaborate PCET reactions remain to be probed.

What comes next?

SLAC coauthor Roberto Alonso Mori framed the broader reach: "By combining complementary X-ray techniques at LCLS, this work provides a uniquely complete view of these coupled processes, opening new opportunities to understand and ultimately control the chemistry that underpins energy conversion and catalysis."

Staff scientist David Hoffman pointed to the upcoming LCLS-II upgrade, which he said will improve signal-to-noise and let researchers tackle real problems in catalysis and energy harvesting.

The findings could eventually inform the design of more efficient catalysts, fuel cells and flow batteries. The DOE Office of Science, Basic Energy Sciences, through the CPIMS and AMOS programs at PNNL, supported the research. The team also used the Environmental Molecular Sciences Laboratory at PNNL.