Knowledge Bank

Helium clusters are the only clusters which are definitely liquid and, since they are predicted to be superfluid, are expected to exhibit unusual properties. Large clusters with $103-104$ atoms (called droplets) are readily produced in nozzle beam expansions of the cold gas. Their mean sizes, size distributions and densities have been measured. On passing through a gas filled pick-up cell they readily absorb one or several molecules or mixtures of atoms and molecules, which agglomerate to complexes. A large number of single molecules and complexes have now been probed in the infra-red and visible by observing the decrease in droplet size resulting from photon absorption stimulated evaporation. In many cases sharp spectral features with line widths of $\approx 100$ MHz have been obeserved. The vibronic spectra of organic molecules reveal sharp zero phonon lines and a phonon wing separated by a gap which provides direct evidence that the droplets are superfluid. The well resolved rotational lines in the infrared indicate, furthermore, that the molecules rotate without friction while retaining the same symmetry as the free molecules. From the rotational line intensities droplet temperatures of 0.37 K for $\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}He$ droplets and 0.15 K for $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}He$ droplets are obtained. The free rotations of embedded molecules, recently demonstrated also for large clusters with up to 16 molecules, have been shown to be a new manifestation of superfluidity [1]. The large differences in the spectroscopic parameters compared to the gas phase have been analyzed in detail for embedded OCS molecules and reveal a surprisingly rich and complex dynamical coupling to the superfluid environment. With this technique, the high resolution spectra of a large number of species including biological molecules, metal clusters and van der Waals clusters - some with sizes and structures not otherwise possible to produce - have been studied. Several examples, which illustrate the advantages over seeded beam cooling, will be presented.