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Extreme ultraviolet and X-ray spectroscopy to understand dynamics beyond the Born Oppenheimer Approximation

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Towards an accurate quantum description of molecular dynamics

EU-funded researchers have blasted complex molecular systems with extreme ultraviolet (XUV) and soft X-ray radiation to probe their dynamics on the quantum level. The study has offered valuable insights into the intricacies of chemical reactions that cannot be described with the Born-Oppenheimer approximation.

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Protons in molecules and atoms are about 1 800 times heavier than the fast-moving electrons. That is why, in most cases, scientists assume that the nuclei move so slowly that electrons remain in their ground state for a given set of nuclei positions. This so-called Born-Oppenheimer approximation has been widely used in quantum mechanical simulations of complex molecular systems. In some cases, however, the Born-Oppenheimer approximation is not applicable. For example, as an atom moves backwards and forwards in a metal, it interacts with other electrons and loses some of its energy to the electron gas. The drag forces exerted by the electron gas on the vibrating atom are non-conservative and the damping of vibrations cannot be described by the Born-Oppenheimer approximation. Within the EU-funded project XBEBOA (Extreme ultraviolet and X-ray spectroscopy to understand dynamics beyond the Born Oppenheimer approximation), scientists looked beyond the Born-Oppenheimer approximation. The team developed advanced tools making use of new femtosecond-light pulses to study metal-organic complexes. Specifically, XBEBOA scientists exploited high harmonic generation, a technique for producing spatially and temporally coherent XUV light as well as light pulses as short as hundreds of femtoseconds. The experimental set-up developed consists of transient grating, time-resolved photoelectron and photoion spectrometers for gas and liquid solutions studies. In the first experiments on the insulator-to-metal phase transition of vanadium dioxide samples, it was possible to distinguish the spectral signatures of electronic and nuclear processes clearly. To differentiate XUV light in the vicinity of excited electrons from probing samples at lower photon energies was a major challenge for ultrafast spectroscopy before the XBEBOA project. In addition, the team was able to create conditions allowing each atom in a buckminsterfullerene (C60) molecule to absorb multiple photons during X-ray pulses of femtoseconds' duration. For this purpose, they employed the free-electron laser of a linac coherent light source. Similar conditions are needed to image proteins and viruses in investigations of biomolecular samples. XBEBOA scientists hope to utilise the experimental set-up beyond the end of the project to calculate the properties of solids, liquids and gases, avoiding crude approximations used in the past to describe their quantum mechanical behaviour.

Keywords

Molecular dynamics, extreme ultraviolet, soft X-ray radiation, Born-Oppenheimer approximation, quantum mechanical

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