Making molecular movies more reliable


Combination of computer simulations and two experiments enables precise observation of nuclear and electronic motion

Photochemical reactions play a central role, for example, in sun protection, light-resistant materials or the generation of energy from light. How the interaction of UV light and matter leads to the splitting of molecules has now been closely observed by an international team, including Philipp Marquetand from the Faculty of Chemistry at the University of Vienna, using a novel approach. By combining two established experimental methods and simulations, it was possible to follow both the nuclear and electron motion of a molecule excited by UV light. The novel method for creating "molecular movies" was published in the scientific journal "Physical Review X".

If a molecule absorbs UV light, it is excited. There is an energy exchange between its nuclei and electrons resulting in structural changes. Molecular dynamics can only be understood indirectly through the experiment. Depending on the method used, in-depth studies have so far been carried out for either the motion of the electrons or that of the nuclei.

Using diiodomethane (CH2I2), an atmosphere-relevant molecule, the physicists and chemists presented a new approach: "When exposed to UV light, an iodine atom breaks away from the molecule and the remaining fragment begins to rotate. By combining spectroscopic and structural analysis methods and theory, we were able to describe the molecular dynamics relatively completely," says Philipp Marquetand from the Institute of Theoretical Chemistry.

Ultrafast motion

The motion of molecules can be studied in the laboratory, for example, by analyzing different signals. Using photoionisation (TRPES, time-resolved photoelectron spectroscopy), a team led by US physicist Thomas Weinacht (Stony Brook) studied how the electrons of diiodomethane behave after being bombarded with UV light.

A second team led by the US physicist Xijie Wang (Stanford) used ultrafast electron diffraction (UED) to investigate the motion of the nuclei in the femtosecond range (1 femtosecond = millionth part of a billionth of a second).

Direct view of nuclear and electron motion

"We simulated the experiments on the computer and compared aspects such as the positions of the nuclei over time and the quantum energy levels of the electrons in our simulation with those from the two experiments. This is the first time that both UED and TRPES have been combined with a theory capable of calculating the observed values in both cases, which has successfully provided a direct view of molecular dynamics," says Philipp Marquetand, who with his Vienna group specializes in the simulation of chemical reactions in real time.

The novel method for creating "molecular movies" is a strong proof of the synergies between experimental methods and theory and could also be an approach for the recently launched research platform ViRAPID (Vienna Research Platform on Accelerating Photoreaction Discovery) - an initiative of chemists, physicists and mathematicians at the University of Vienna.

S E R V I C E: Spectroscopic and structural probing of excited-state molecular dynamics with time-resolved photoelectron spectroscopy and ultrafast electron diffraction, in Phys. Rev. X, Yusong Liu, Spencer L. Horton, Jie Yang, J. Pedro F. Nunes, Xiaozhe Shen, Thomas J. A. Wolf, Ruaridh Forbes, Chuan Cheng, Bryan Moore, Martin Centurion, Kareem Hegazy, Renkai Li, Ming-Fu Lin, Albert Stolow, Paul Hockett, Tamás Rozgonyi, Philipp Marquetand, Xijie Wang, and Thomas Weinacht, 

More reliable and direct view of nuclear and electron motion (Copyright: Philipp Marquetand)