Femtosecond X-ray laser pulses can reveal new details of the mechanisms of surface-catalyzed reactions, reports an international team of researchers using the X-ray free-electron laser at SLAC National Accelerator Laboratory’s Linac Coherent Light Source (Science 2015, DOI: 10.1126/science.1261747). The method probes how electronic structure evolves in bimolecular interactions, resolving events that happen within hundreds of femtoseconds. The experiments were led by Anders Nilsson of SLAC and Stockholm University and Henrik Öström and Lars Pettersson of Stockholm University. The scientists studied carbon monoxide oxidation on a ruthenium surface. They used a so-called pump-probe approach, applying a 400-nm laser as the “pump” to excite the molecules, then X-ray pulses as the “probe” to generate X-ray absorption spectra of the reactants CO and O as they form the product CO2. Coupling the experimental data with theoretical modeling, the researchers suggest that the reactants go through a first transition state in which the reactants reorient on the surface and come together to form a long OC—O bond. The species then goes through a second transition state that involves a bent CO2 molecule with a shorter, but still elongated, OC–O bond.