Bridging the gap in molecular dynamics

Interaction between electrons and between electrons and nuclei is described by a many-particle Schrodinger equation that is generally too complex to be solved. The Born–Oppenheimer approximation (BOA) — which simplifies electron–nuclear interactions — breaks down when two potential energy surfaces cross each other in a conical intersection (CI). Locating and characterising these CIs are essential to understanding a wide range of chemical reactions.

The EU-funded project XBEBOA is studying molecular dynamics in certain chemical elements and compounds that cannot be described by the BOA framework. XBEBOA is advancing state-of-the-art ultrafast pump–probe spectroscopy by implementing novel time-resolved setups that use XUV and soft X-ray radiation to probe ultrafast molecular dynamics.

To date, main project achievements include construction and commission of a XUV transient grating setup and its application to the photo-induced phase transition in vanadium dioxide. The setup is flexible and is based on a commercial laser system. XUV light is generated by high-harmonic generation in noble gases and used for time-resolved spectroscopy. An in-line monitoring tool has been developed that measures the XUV flux on a shot-by-shot basis and can be used in data acquisition to correct fluctuations.

First results on vanadium dioxide samples show that the XUV light in vicinity of the M edge — excited electrons — is clearly distinct from off-resonance probing at lower photon energies.

Furthermore, a time-resolved photoelectron spectroscopy (PES) setup has been installed and commissioned. This technique has been used to study the gas-phase molecular samples after excitation with XUV light, overcoming limitations of state-of-the-art PES. The results obtained from perylene have shown important discrepancies between XUV and multiphoton infrared modes.

Understanding the molecular dynamics in CIs should further reveal the most fundamental processes of our very existence and improve strategies in clean energy research. The setup description and project results have been published in peer-reviewed journals, and at least other six scientific papers are on the way.