“To identify and characterize short-lived intermediate states in chemical reactions on metal surfaces has long been a dream,” says Henrik Öström at the Department of Physics, Stockholm University, who is part of the international research team that carried out the study. “With the new free electron x-ray laser at SLAC, we have shown that dreams can become reality and managed to identify a short-lived intermediate state when the bindings of CO molecules to a metal surface are broken or created.
SLAC National Accelerator Laboratory was long a flagship of particle physics, where electrons were accelerated to nearly the speed of light in a three-kilometer-long linear accelerator. Now the accelerator has been rebuilt to generate, instead, powerful ultra-short (10-100 femtosecond) pulses of x-ray beams with a wavelength that makes it possible to examine the surroundings of individual atoms in a molecule. The pulses are sufficiently short to provide a snapshot of the electron distribution around the atom. By varying the delay between the start of a reaction and when the distribution of electrons is monitored with the x-ray pulse, the scientists can create a suspended-time image of changes in the course of the reaction.
“A first challenge was whether the incredibly powerful pulse would destroy the sample,” explains Anders Nilsson, a professor of synchrotron-light physics at SLAC and an adjunct professor at Stockholm University. “However, it turned out to be entirely possible to adjust the experiment in a way that enabled us to make our measurements.”
In the experiment, CO molecules were dosed onto a metal surface of ruthenium, which is used in automobile catalytic converters, for instance. CO binds strongly to the surface but can be made to let go by heating up the surface, which was done with a pulse from an optical laser. By starting the reaction for all the molecules at the same time, the team got a sufficient number of molecules to simultaneously enter a state where they have almost let go of the surface but still have a weak binding to it. From this short-lived state, the molecules can then continue out into gas phase or renew their bond to the surface when the surface cools down again.
“Scientists have long speculated whether such a state, a so-called ‘precursor’, exists. The new experiment is the first to directly show its existence,” says Lars G. M. Pettersson at the Department of Physics, Stockholm University. These studies will now go on to more complex reactions of interest to generating synthetic fuels, among other applications.
Besides the researchers from Stockholm University, scientists from Stanford University, SLAC, the University of Hamburg, the Technical University of Denmark, the Helmholtz Center in Berlin, and the Fritz Haber Institute in Berlin contributed to the study.
Röntgenlasern undersöker förändringen i elektronstrukturen då CO molekyler desorberar från en metallyta av rutenium. Ungefär 30% av molekylerna pumpas med hjälp av en femtosekunds optisk laser upp från det ytbundna (”chemisorbed”) tillståndet till ett transient mellantillstånd (”precursor”) där de växelverkar svagt med ytan. Genom att undersöka molekylerna med röntgenlasern med olika fördröjning kan man visa att tidsskalan för att nå precursortillståndet är några pikosekunder och att de befinner sig där några tiotal pikosekunder innan de antingen lämnar ytan helt eller återvänder till ytan.
The free electron x-ray laser makes it possible to follow, in real time, how bonds in a molecule are changed and broken.

Further information:
Henrik Öström at the Department of Physics, Stockholm University, phone: +46 (0)8 5537 8641 mobile: +46 (0)73 035 9150 e-mail: ostrom@fysik.su.se
Lars G.M. Pettersson at the Department of Physics, Stockholm University, phone: +46 (0)8 5537 8712 mobile: +46 (0)70 495 1990 e-mail: lgm@fysik.su.se
Anders Nilsson at SLAC and Stockholm University, phone: +1 650 926 2233 e-mail: nilsson@slac.stanford.edu