Large 4He, 3He or mixed droplets with 10³ - 10⁴ atoms are readily produced in nozzle beam expansions of the cold gas. Their mean sizes, size distributions and densities have all 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 may agglomerate in the interior to complexes. Heliophobic species such as the open shell alkali atoms and the alkaline earth atoms remain at the surface, whereas closed shell atoms and molecules, which are heliophilic, reside in the interior of the droplets. Embedded heliophilic molecules and van der Waals complexes have been shown to exhibit sharp spectral features in the infra-red, visible and UV by observing the depletion in droplet size resulting from the evaporation following photon absorption [1].

In the vibronic spectra of organic molecules the phonon wing is separated by a gap from the sharp zero phonon lines thereby providing direct evidence that the droplets are superfluid [1]. The well resolved rotational lines in the infrared indicate, moreover, that molecules such as OCS rotate virtually without friction but with about a factor 3 greater moment of inertia. The rotational line intensities indicate droplet temperatures of 0.37 K for pure ⁴He droplets and 0.15 K for both pure ³He droplets as well as for mixed ⁴He/³He droplets with an inner ⁴He cluster core. The free rotations have been shown to be a new microscopic manifestation of superfluidity [2].

With the aim of understanding the increase in the moments of inertia recent spectroscopic experiments have been designed to study in detail the local interactions of the molecule with the first shell of He atoms. By adding one-by-one H₂, D₂ or HD molecules, which replace the first shell atoms, details on their structures, dynamics and exchanges with the other atoms in the superfluid have been obtained.

Large (OCS)-(pH₂)15 clusters have also been created inside the droplets and have been observed to undergo a transition to a superfluid state on cooling from 0.38 to 0. 15 K [3].  Other clusters grown inside He droplets appear to have unusual structures not found in gas phase expansions [4].

These experiments illustrate how inside He droplets new exotic states of low temperature matter can be produced with sizes intermediate between the molecules and small clusters studied in gas phase spectroscopy and the macroscopic systems characteristic of low temperature physics. The new technique thus provides new perspectives for both of these mature fields.

1.  J.P. Toennies and A.F. Vilesov, Ann. Rev. Phys. Chem. 48, 1 (1998).
2.  S. Grebenev, J.P. Toennies and A.F. Vilesov, Science 279, 2083 (1998).
3.  S. Gebenev, B.Sartakov, J.P. Toennies and A.F. Vilesov, Science, in print.
4.  K. Nauta and R.J. Miller, Science 283, 1895 (1999).

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