Scientists Observe Energy from Light Flowing Through Atomic Ripples in a Molecule

A research team succeeded in watching energy from light flow via atomic ripples in a molecule. Such insights could provide new methods for developing a class of materials capable of improving efficiency and reducing the size of memory storage devices and solar cells.

An international team of Researchers working at the Department of Energy’s SLAC National Accelerator Laboratory used an X-ray laser to watch  “molecular breathing”, in real time and unparalleled detail, as part of a landmark for analyzing a class of chemical reactions applicable to novel memory storage devices and solar cells. Molecular breathing refers to waves of subtle in-and-out motions of atoms.

These ripples of motion, observed with SLAC’s Linac Coherent Light Source (LCLS), enabled the Researchers to study how the exchange of energy takes place between light and electrons and how this leads to tension and ultimately motion of atoms in an iron-based molecule that is a model for converting light into electric energy and switchable small molecular magnets.

The research team, in a paper featured in Nature Communications, stated that these real-time, high fidelity measurements of ultrafast energy redistribution can provide vital information for understanding the function of several biological, physical and chemical light-induced phenomena.

It’s a significant leap in experiment sensitivity that now allows us to see more of what’s happening. We’re zooming into the details of molecules as we achieve better and better resolution in both space and time.

Diling Zhu, Staff Scientist, SLAC

The molecule they studied comprises of a central iron atom fixed to three double-ringed structures called bipyridines.

In order to see it “breathe”, the Researchers started by initially hitting the molecule with laser light and this was immediately followed up with an X-ray laser pulse to analyze any changes that occurred.

An electron was excited by the laser light in the central iron atom, which was transported to one of the fixed bipyridine structures. The electron flipped the magnetism of the iron after it returned to the iron atom 100 femtoseconds, or quadrillionths of a second later. This resulted in the molecule expanding, thus setting off a breath-like oscillation via the whole structure.

Earlier measurements in experiments with optical lasers indirectly exposed these motions, and it was believed that the bending of the bipyridine attachments played a role in contributing to the molecular motion.

However, this experiment using an increasing number of direct signals from X-rays proved that this explanation was not correct. With each X-ray pulse lasting only for 50 femtoseconds, the Researchers were able to observe the electronic excitation by light and also the following breathing process at intervals that are much shorter than ever before and thus attain a concise picture in real time.

Researchers hope to use the insights gained from molecular breathing for enhancing technologies such as memory storage and dye-sensitized solar cells.

Sensitized solar cells are considered to be a promising alternative in the future for low-priced but efficient devices, however their absorbing dyes are made up of costly rare metals such as ruthenium. Scientists aim at using cost-effective iron-based compounds instead, but magnetic switching that encourages the molecular breathing arrests the flow of electrical current across a solar cell.

We see two competing processes in the molecule and their relation to molecular structure. With this information, we may find ways to change the molecular structure in order to favor the usable process for potential technical applications.

Henrik Lemke, Staff Scientist at SwissFEL’s Paul Scherrer Institute in Switzerland and Former Staff Scientist at SLAC

Lemke is Lead Author of the study, which also included Researchers from Sweden, Denmark, Italy and France, as well as from SLAC.

For other applications, the switch is actually desirable, so we could create a molecular memory system. In memory storage devices, a reversible process could enable us to write and store data with the material.

Henrik Lemke, Staff Scientist at SwissFEL’s Paul Scherrer Institute in Switzerland and Former Staff Scientist at SLAC

The experiment marks a major step forward in the potential to visualize molecular dynamics at LCLS's X-ray Pump Probe instrument, which was initially commissioned in 2010. To produce sharper images of the molecular motion, Scientists at LCLS have come up with new methods for distributing samples into the path of the X-ray laser beam and they have also developed special data analysis techniques to account for different fluctuations that can in fact blur the experiment.

The advancements also point out that Researchers are presently capable of collecting higher quality data in less time. Scientists at LCLS will now be able to obtain information within just a few minutes, when compared to the previous process in which it took weeks to collect information.

LCLS is a DOE Office of Science User Facility.