The head of the Femtosecond Dynamics group at the Institute of Experimental Physics at Graz University of Technology Markus Koch and his team develop new methods for time-resolved femtosecond laser spectroscopy to investigate ultrafast processes in molecular systems. The group succeeded in observing a photo-induced reaction of an atom for the first time in 2018in a special environment, namely inside a superfluid helium nanodroplet. For their investigations, the researchers placed a single indium atom in the droplet and analyzed the reaction of the atom using the excitation-query principle (pump-probe). The atom was excited with an ultra-short laser pulse, whereupon the helium environment in the femtosecond range (10-15 seconds) adapted to the excited atomic state. A delayed second laser pulse followed this development and provided information on the behavior of the system.

Successful next step

With the same technology, Koch, together with his institute colleagues Bernhard Thaler and Pascal Heim and with the participation of Miriam Meyer, master’s student in technical physics, visualized the movement of molecules inside a helium nanodroplet for the first time. To this end, the quartet introduced two indium atoms into the droplet, where they combined to form a molecule. A vibration was triggered in the molecule by photoexcitation, the development of which was observed with the pump-probe method described above. Two results are of particular importance for the researchers: Firstly, they were able to demonstrate that it is generally possible with such experiments to observe ultrafast intramolecular processes – i.e. processes that take place in an excited molecule.

Helium has little influence on the embedded molecule

Secondly, the group found that helium has a much weaker influence on molecular movement than conventional solvents such as water or methanol. As a rule, intramolecular processes are influenced by their interactions with the environment. With conventional solvents, however, this interaction is so strong that intramolecular processes cannot be observed, as Bernhard Thaler explains: “Fragile molecules in particular often break after being excited or their vibrations are disturbed. This is demonstrably different in the drop of helium. There are temperatures of 0.4 Kelvin (note minus 272.75 degrees Celsius). As a result, the influence on the embedded molecule is much less and it is possible to observe the intramolecular processes closely and to examine even very fragile molecules. ”

Markus Koch wants to extend the method to complex molecules

“We see great potential in helium nanodrops because they offer wonderful opportunities for creating molecular systems,” explains Koch, explaining why he and his team are increasingly using this method for femtosecond studies. In the next step, the Femtosecond Dynamics group now wants to create more complex structures and observe how this changes the molecular vibration. “Indium molecules are very simple. In the future, we want to look at technologically relevant and at the same time more complex molecules and gradually approach molecular engineering. ”The process could be used in the long term to develop new materials – for organic electronics, for example.  

The research was published in Graz University of Technology