Observing electron clouds, scientists could improve chemical processes
With the help of an X-ray laser, a team of international researchers-including scientists from European XFEL, a new major science facility in the Hamburg metropolitan area, DESY, and the Max Planck Institute for Biophysical Chemistry-has looked more precisely than ever before into the electron cloud, a bunch of charged tiny particles orbiting molecules.
The team managed to document changes in the states of electrons in a similar way to how pictures taken at different times can be assembled to a movie. They were also able to get information comparable to razor-sharp images of even very short-lived transition states that other methods miss. Intermediate states exist on time scales ranging from several femtoseconds (fs, quadrillionths of a second) to ten thousand times longer, and each is decisive for the subsequent course-and final outcome-of chemical reactions. The researchers' work, which is an important step towards atomic- and electronic-scale movies of chemical reactions, has been published in the journal Nature.
"You may imagine the intermediate states as branches in a tree", says Christian Bressler, leading scientist at European XFEL and one of the authors of the paper. "Looking at them from below, we know already quite well what these branches look like, but we do not know their three-dimensional shape." If an apple falls from the tree hitting the branches below, the way the apple would bounce from branch to branch represents the progression of the chemical reaction, with the three-dimensional shape of the branches determining its course, and more importantly, its final destination, or outcome.
"We know where the apple finally will drop down, but we can't see why it winds up there", Bressler says. "But it's this path that also determines the efficiency of the reaction. It is our long-term goal to optimize reactions, using for example light pulses tailored such that we can influence this path, as to make the reaction most efficient and to reach the wished products. The present experiment permitted us to understand a reaction's path for the first time."
In this experimental episode, playing the starring role is what is called an iron coordination complex, a molecule with a central metal atom and organic ligands, a structure that is similar to central components of haemoglobin or chlorophyll. The scene was taken at the world's currently most powerful X-ray free-electron laser, the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory in Menlo Park, California. A normal optical laser transferred the molecule to a higher energy, also called an excited state, thus initiating the first step of a light-induced chemical reaction such as photosynthesis or biochemical processes in the retina that enable sight.
The excited states of the central atom are heavily influenced by the changing ligand structure and are usually described by main shell, orbital, and spin-characteristics called quantum numbers. The quantum numbers of the reactants and products of a chemical reaction are generally well known, but those of the intermediate states, which determine the outcome of the reaction, were not clear to scientists.
To catch these intermediate states, the researchers took a brutal approach. They fired the hard X-ray flashes of the LCLS, one of the two lightsources which are currently capable of producing such extremely short and bright flashes, at the freshly excited molecules. These probing flashes blasted out the innermost bound electron of the iron atom in the complex. An electron from one of the outer shells of the iron atom then dropped into the vacancy, emitting X-radiation. The spectrum obtained from those X-rays is representative of the molecular quantum states. The researchers repeated these measurements, taking snapshots at different times, in the same way as pictures can be assembled into a motion picture. That motion picture-like result showed the molecule's journey, which lasted just a few hundred femtoseconds, through two now clearly identified excited states to the final state. "The experiments benefitted from the unique capabilities of X-ray lasers which have been emerging over the last few years," explains co-author Katharina Kubicek, scientist at DESY and the Max-Planck-Institute for Biophysical Chemistry who is currently a Peter Paul Ewald fellow working at the PULSE Institute at SLAC. "In contrast to many conventional techniques, the X-ray flashes of LCLS specifically probe the situation at the iron center of the molecule where interesting processes are taking place."
"The measurements alone do not yet give us the quantum states of the electrons, but comparing them with model spectra, we could determine with a probability of 95 percent that these states were visited by the reacting molecule in action", explains co-author Kelly J. Gaffney from SLAC.
"This is an important step towards an ultimate high-speed camera for molecules. While other types of experiments will allow us to show the atomic movements, we now were able to cover the subtle energetic and magnetic details of the electron clouds driving these molecular structure changes", says Wojciech Gawelda, a scientist at European XFEL and also an author on the publication. "As in the example of the apple falling from the tree and bouncing from branch to branch until it finally falls into the grass, we could follow the exact course of this particular reaction. This offers the potential to eventually control-thus optimize the outcome during-the course of a chemical reaction." With such a camera, Bressler adds, scientists could stringently test green chemistry, molecular systems that efficiently generate fuel from solar energy or destroy air pollutants in an environmentally friendly way.
A new scientific instrument at the European XFEL from 2017 on will allow making real ultrahigh-speed movies with even sharper resolution. "Our Femtosecond X-Ray Experiments instrument will enable scientists not only to do research as described now in Nature, but will also offer a suite of individual cameras for different aspects of molecular dynamics to look into even more details about the moving atoms as well as about the electrons within femtosecond time frames", Bressler says. "This would be the first modern molecular microscope that would capture even the faintest movements in the molecule in ultraslow motion."