Ultrafast dissociation of molecules studied at BESSY II
The X-ray photons trigger a ‘molecular catapult effect’: light atomic groups are ejected first, similar to projectiles shot from a catapult, while the heavier atoms – bromine and chlorine – separate much more slowly. The image was printed on the cover of "The Journal of Physical Chemistry Letters". © The Journal of Physical Chemistry Letters
For the first time, an international team has tracked at BESSY II how heavy molecules – in this case bromochloromethane – disintegrate into smaller fragments when they absorb X-ray light. Using a newly developed analytical method, they were able to visualise the ultrafast dynamics of this process. In this process, the X-ray photons trigger a "molecular catapult effect": light atomic groups are ejected first, similar to projectiles fired from a catapult, while the heavier atoms - bromine and chlorine - separate more slowly.
When X-rays hit molecules, they can knock electrons out of certain orbitals and into extremely high-energy states, breaking chemical bonds. This often happens ultra rapidly, in just a few femtoseconds (10-15 s). While this phenomenon has been studied in light molecules such as ammonia, oxygen, hydrochloric acid or simple carbon compounds, it has hardly been studied in molecules with heavier atoms.
A team from France and Germany has now studied the rapid decay of molecules containing halogens. They focused on a molecule in which bromine and chlorine atoms are linked by a light bridge - an alkylene group (CH2). The measurements were made at the XUV beamline of BESSY II.
The absorption of the X-rays caused molecular bonds to break, creating ionic fragments that could be analysed. The scientists were able to produce a visualisation from the measurement data. It shows how the atoms move in the fleeting intermediate states just before the bonds break. To do this, the team developed a new method of analysis called IPA (Ion Pair Average) and combined it with ab initio theoretical calculations to reconstruct the processes.
The results show that light groups of atoms such as CH2 are ejected first, while the heavier atoms - bromine and chlorine - are left behind and therefore separate more slowly. Interestingly, this catapult-like behaviour only occurs at certain X-ray energies. Theoretical simulations, in agreement with experimental observations, emphasise the crucial role of vibrations of the lighter groups of atoms in triggering these ultrafast reactions.
“This study highlights the unique dynamics of molecular dissociation upon X-ray irradiation," says Dr Oksana Travnikova (CNRS, Université Sorbonne, France), first author of the study now published in J. Phys. Chem. Lett. In particular, it shows that the catapult-like motion of light groups initiates the separation of heavy fragments, a process that unfolds in a remarkably short time. These findings could deepen our understanding of chemical reactions at the molecular level and how high-energy radiation affects complex molecules.
- The future of corals – what X-rays can tell usThis summer, it was all over the media. Driven by the climate crisis, the oceans have now also passed a critical point, the absorption of CO2 is making the oceans increasingly acidic. The shells of certain sea snails are already showing the first signs of damage. But also the skeleton structures of coral reefs are deteriorating in more acidic conditions. This is especially concerning given that corals are already suffering from marine heatwaves and pollution, which are leading to bleaching and finally to the death of entire reefs worldwide. But how exactly does ocean acidification affect reef structures?
Prof. Dr. Tali Mass, a marine biologist from the University of Haifa, Israel, is an expert on stony corals. Together with Prof. Dr. Paul Zaslansky, X-ray imaging expert from Charité Berlin, she investigated at BESSY II the skeleton formation in baby corals, raised under different pH conditions. Antonia Rötger spoke online with the two experts about the results of their recent study and the future of coral reefs.
- Energy of charge carrier pairs in cuprate compoundsHigh-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.
- Electrocatalysis with dual functionality – an overviewHybrid electrocatalysts can produce green hydrogen, for example, and valuable organic compounds simultaneously. This promises economically viable applications. However, the complex catalytic reactions involved in producing organic compounds are not yet fully understood. Modern X-ray methods at synchrotron sources such as BESSY II, enable catalyst materials and the reactions occurring on their surfaces to be analysed in real time, in situ and under real operating conditions. This provides insights that can be used for targeted optimisation. A team has now published an overview of the current state of knowledge in Nature Reviews Chemistry.