Nano droplet of liquid Helium are very resistent towards energy disturbances
Paper by Marcel Mudric, IFA et al. in Nature Communications 8. January 2020
Nanobubbles in nanodroplets -- probing the ultrafast relaxation of superfluid helium using a tunable extreme ultraviolet free-electron laser.
Helium nanodroplets are intriguing quantum systems featuring extraordinary properties such as an extremely low temperature (0.37 K) and frictionless motion (superfluidity). Due to these properties, and helium droplets being chemically inert and completely transparent to infrared and visible light, they are usually considered as the "ideal test glass" for probing embedded molecules by spectroscopy. But how does a superfluid droplet itself react when it is directly excited by a laser pulse? This question was answered by an international research team lead by M. Mudrich and Frank Stienkemeier taking advantage of the world's first and only seeded free-electron laser FERMI in Trieste, which provides wavelength-tunable extreme ultraviolet pulses.
The electronic energy risses abruptly, but after only half a pico second evertything looks the same again. Illustration from the paper: M. Mudrich.
Backed by high-level model calculations, the researchers identified three elementary relaxation steps: Ultrafast electron localization, electronic decay into metastable states, and the formation of a void bubble, which eventually bursts at the droplet surface, thereby ejecting a single excited helium atom. These results help understanding how nanoparticles interact with energetic radiation, as it happens when single nanoparticles are directly imaged for example at the new European XFEL facility.
The paper titeld "Ultrafast relaxation of photoexcited superfluid He nanodroplets" can be found here.