Tag: X-ray flashes

Researchers watch chemistry unfold atom by atom

Researchers watch chemistry unfold atom by atom

Study suggests a new way to follow electronic and vibrational dynamics in real time

Researchers have captured how a molecule redistributes energy after absorbing light, differentiating the roles of individual atoms in the process. They used X-ray flashes from the European XFEL to show that different atoms of the same molecule can reveal entirely different aspects of the process. The study provides clear evidence that excitation by light can enhance an atom’s sensitivity to motion of nearby atoms. The new method for following ultrafast chemical reactions at the atomic scale and in real-time can help understanding photostability in DNA, energy flow in light-harvesting materials, and other fundamental processes driven by light.

The team investigated 3-fluoropyridine, a small ring-shaped molecule. When the molecule absorbs light like a short pulse from an ultraviolet laser, it is promoted into an electronically excited state and rapidly distorts out of its original planar shape. It then passes through a so-called conical intersection: a short-lived but crucial crossing point where movements of electrons and the atoms’ cores become strongly coupled. After this point, the molecule returns to the ground state. At that moment, electronic energy is converted into vibrations. The researchers found that this conversion leaves distinct fingerprints at different atomic sites: the fluorine atom acts as a clean marker of vibrational relaxation, while the nitrogen atom, which is more directly involved in the excitation, reflects an intertwined response of electron redistribution and structural motion. “We can now see that not every atomic site tells the same story in the signals we capture from our X-ray pulses,” says Antonio Picón from the Instituto de Ciencia de Materiales de Madrid Consejo Superior de Investigaciones Científicas (ICMM-CSIC), co-author of the study. “Some atoms report where the charge is going, while others reveal how the whole molecule vibrates.”

To observe this process, the team used time-resolved X-ray photoelectron spectroscopy (tr-XPS) at the Small Quantum Systems instrument (SQS) of European XFEL. An ultraviolet laser pulse first excited the molecules, and a precisely delayed soft X-ray pulse then ionized them by removing deeply bound electrons from either the nitrogen or fluorine atoms. By measuring the energy of these emitted electrons at many different time delays, the scientists reconstructed how the local chemical environment evolved over the course of just a couple of picoseconds (trillionths of seconds). To interpret the data, the team developed advanced simulations and models.

Read more on the European XFEL website

Image: Experiment on 3-fluoropyridine: a UV pulse (purple beam from the left) excites the molecule, and a delayed X-ray pulse (white beam from the lower right) probes the nitrogen atom (purple sphere). The measurement reveals light-induced charge redistribution (purple cloud) from the perspective of a core-photoelectron from the nitrogen site (concentric rings). The fluorine atom (green sphere) can also be probed and acts mainly as a marker of vibrational dynamics.

Credit: European XFEL/Enrique Sahagun

How Molecules Break and Form Bonds

How Molecules Break and Form Bonds

Researchers at European XFEL in Germany have tracked in real time the movement of individual atoms during a chemical reaction in the gas phase. Using extremely short X-ray flashes, they were able to observe the formation of an iodine molecule (I₂) after irradiating diiodomethane (CH₂I₂) molecules by infrared light, which involves breaking two bonds and forming a new one. At the same time, they were able to distinguish this reaction from two other reaction pathways, namely the separation of a single iodine atom from the diiodomethane, or the excitation of bending vibrations in the bound molecule. The results provide new insights into fundamental reaction mechanisms that have so far been very difficult to distinguish experimentally.

So-called elimination reactions in which small molecules are formed from a larger molecule are central to many chemical processes—from atmospheric chemistry to catalyst research. However, the detailed mechanism of many reactions, in which several atoms break and re-form their bonds, often remains obscure. The reason: The processes take place in incredibly short times—in femtoseconds, or a few millionths of a billionth of a second.

An innovative experimental approach was now used at the SQS instrument at European XFEL to visualize such reaction dynamics. The researchers irradiated diiodomethane molecules with ultrashort infrared laser pulses, which triggered the molecular reactions. Femtoseconds later, intense X-ray flashes shattered the molecules, causing their atomic components to fly apart in a “Coulomb explosion.” The trajectories and velocities of the ions were then recorded by a detection device called the COLTRIMS reaction microscope (COLd Target Recoil Ion Momentum Spectroscopy)—one of the detection instruments at the SQS experimental station that is made available to users.

“Using this method, we were able to precisely track how the iodine atoms assemble while the methylene group is cleaved off,” explains Artem Rudenko from Kansas State University, USA, the principal investigator of the experiment. The analysis revealed that both synchronous and asynchronous mechanisms contribute to the formation of the iodine molecule—a result that was supported by theoretical calculations.

Remarkably, “Although this reaction pathway only accounts for about ten percent of the resulting products, we were able to clearly distinguish it from the other competing reactions,” explains Rebecca Boll from the European XFEL’s SQS (Small Quantum Systems) instrument in Schenefeld near Hamburg. This was made possible by the precise selection of specific ion fragmentation channels and their time-resolved analysis.

Read more on European XFEL website