Ultrafast reaction of superfluid helium triggered by ultraviolet laser pulses, The team observed super-fast responses of helium nanoparticles after excitation with ultraviolet radiation (XUV) using an electron-free real-time laser.

Lasers that produce high-intensity and ultra-short XUV and X-ray pulses offer researchers new opportunities to study the basic properties of matter in detail. In many such experiments, material samples in the nanometer range are very interesting.

Some scientists use helium droplets no bigger than a few nanometers to transport and study embedded molecules and molecular nanostructures. Helium droplets are ideal for this purpose because they have extraordinary properties.

At very low temperatures only 0.37 degrees above absolute zero, they move smoothly and are therefore considered excessive. In addition, helium droplets are usually inert to molecular chemical processes that are embedded and are fully transparent to infrared and visible light.

The team wanted to find out how one of these ultra-liquid droplets responded to the direct attack from the intense XUV laser pulse. In Trieste (Italy), the researchers used the world’s first and only laser-free FERMI electron, which delivers high-intensity XUV pulses with wavelengths determined by a team.

Using model calculations, the researchers identified three steps in elementary reactions: very fast electron localization, populations of metastable states, and the formation of bubbles that eventually burst onto the droplet surface and release excited excited helium atoms.

For the first time, we were able to directly follow this process in ultra-liquid helium, which took place in a very short time, according to researchers.

The results help to understand how nanoparticles interact with energy and then decompose. This is important information for working on direct imaging of individual nanoparticles, because they are carried out on new intensive radiation sources such as the European XFEL X-ray laser in Hamburg.