A New Effect in Ultrafast X-ray Scattering

Light triggers many important chemical reactions. These include photosynthesis (converting sunlight to chemical energy), human vision (detecting photons via light-induced changes in molecules), and new technologies such as photodynamic therapy for cancer, photocatalysis, fluorescent tags for healthcare diagnostics, and photovoltaics. Light-triggered processes in molecules are difficult to study experimentally and involve a complex interplay of concerted changes in molecular structure and rapid rearrangements of the electrons in the molecule.

Conical intersections play a decisive role for the outcome of photochemical reactions, analogous to that of a transition state in standard ground-state chemistry. These are regions on photochemical pathways where molecules can transition efficiently between electronic states. Being able to map the path of molecules through conical intersections would open avenues to controlling photochemical reactivity via modification of excited state dynamics. To achieve this we must simultaneously observe the electronic characteristics of the molecule and the corresponding changes in molecular structure. The challenge is compounded by the short timescales involved, on the order of femtoseconds. Notably, there are as many femtoseconds in a second as there are seconds in 30 million years. In contrast, standard techniques for structural determination require long observation times.

New facilities known as X-ray Free-Electron Lasers (XFELs) deliver extremely short pulses of intense high-energy x-ray photons, making completely new types of measurements possible. In recent work, we have demonstrated that we can track the changes in molecular structure in excited molecules and, in separate experiments, detect the nearly instantaneous re-arrangement of electrons when molecules absorb light. Exploiting these advances, the proposed project will develop measurements that track the motion of electrons alongside the motion of the nuclei, allowing conical intersections to be identified, and the structure of molecules at conical intersections to be determined. The resulting experimental technique will yield a powerful tool for fundamental research and provide images of electrons and nuclei that can be used to customise photoactive molecules, ultimately contributing to new technologies in catalysis, new cancer treatments, and energy harvesting from sunlight.