Superfluid helium nanodroplets: 2006 Benjamin Franklin Medal in Physics presented to Giacinto Scoles and J. Peter Toennies

At very low temperatures, helium becomes superfluid, with properties significantly different from those of familiar substances. Although superfluid helium has been the subject of extensive investigations, until recently no practical applications have been found that would utilize its unusual properties. The main reason was that macroscopic samples of superfluid helium expel all other atoms or molecules. Only in the 1990s, Scoles and collaborators demonstrated that one can embed “impurities”, atoms and molecules, in low-temperature helium if it is in the form of small droplets. Such droplets contain only a few thousands atoms and therefore are called “nanodroplets”. Furthermore, Scoles and coworkers were able to measure spectra of the impurities. Subsequently, Toennies and collaborators have found that the spectral lines are very sharp, almost as sharp as in the gas phase, and demonstrated that this sharpness has to be due to the superfluidity of helium in the nanodroplets. The newly created technique of helium-nanodroplet spectroscopy allowed investigations of molecules or clusters that are unstable in gas phase and has significantly increased our understanding of superfluid helium at microscopic level.