Researchers Observe Ultrafast Processes of Single Molecules in Liquid Helium for the First Time

by Christoph Pelzl, communication and marketing, TU Graz,

A team at the Femtosecond Dynamics research group at the Institute of Experimental Physics at TU Graz, in Graz, Austria, has visualized the movement of single, isolated molecules inside a helium droplet for the first time. The findings, which have implications for future experiments in visualizing processes within excited molecules, were published in Physical Review Letters on March 23.

The TU team, led by Markus Koch, included Miriam Meyer, Bernhard Thaler and Pascal Heim, who demonstrated that photo-induced processes can be observed inside a helium nanodroplet—a nanometer-sized droplet of superfluid helium that serves as a quantum solvent. The researchers placed a single indium atom inside the droplet and analyzed the reaction of the system with the pump-probe principle. The atom was excited with an ultrashort laser pulse, triggering the rearrangement of the helium environment within femtoseconds—10 to 15 seconds. A time-delayed laser pulse probed this development and provided information on the behavior of the system.

Now the researchers have formed an indium dimer molecule inside a helium droplet by loading it successively with two indium atoms. They then triggered a vibration in the molecule by photoexcitation and observed the movement of the nuclei in real time with the same pump-probe technique.

The researchers consider two aspects of the experiment as particularly important: first, it demonstrates that such experiments are able to observe ultrafast intramolecular processes like that which occur within an excited molecule. Second, the group discovered that the influence of superfluid helium on molecular vibrations is significantly weaker than with conventional solvents, such as water or methanol.

Intramolecular processes are usually influenced by interactions with the environment and, in conventional solvents, this interaction is so strong that intramolecular processes cannot be observed. “The quantum fluid helium, which has a temperature of only 0.4 K is truly special, as the perturbation on the embedded molecule is very low,” said Thaler. “Additionally, fragile molecules, which often break apart in other techniques, are stabilized due to the cooling mechanism and can now be investigated.”

In their next step, the Femtosecond Dynamics group will aim for more complex systems. “We see great potential in helium nanodroplets because they offer wonderful opportunities for creating molecular systems,” explains Koch. “The structure of indium molecules, which we used as a model system, is very simple. In the future, we want to look at technologically relevant molecules—which are more complex. I consider this a promising approach to molecular engineering, where future materials are developed by manipulating the quantum behavior of their molecular constituents.” ■