Ultrafast dissociation of molecules studied at BESSY II

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.