We have built an apparatus capable of attaching a molecule to a liquid-helium cluster and obtaining its infrared spectrum. Clusters made of a few thousand atoms and containing an ${\mathrm{SF}}_6$ molecule show two absorptions located at 945.8 and 946.1 ${\mathrm{cm}}^{\mathrm{-}1}$ which indicate that the guest molecule is located near the surface of the cluster. If the cluster contains two ${\mathrm{SF}}_6$ molecules, the spectrum obtained is that of a slightly perturbed ${\mathrm{SF}}_6$ dimer.

• Received 10 February 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.933