Molecular orientation in superfluid liquid helium droplets: high resolution infrared spectroscopy as a probe of solvent–solute interactions

Abstract

Reported here are three aspects of molecular stereochemistry associated with polar molecules in an exotic solvent, namely, superfluid liquid helium. Infrared spectroscopy is used to study the nature of the solvent–solute and solute–solute interactions in this unique environment. First, we use an external dc electric field to orient the helium solvated polar molecules in the laboratory frame of reference and we characterize this orientation using pendular state spectroscopy. Second, we present results that demonstrate non-equilibrium self-assembly of polar molecules into linear chain structures, due to the mutual orientation imposed by the associated dipole–dipole interactions. Finally, the spectroscopic results provide information on the anisotropic interactions experienced by the solvated molecules, which result in significant molecular orientation in the droplet frame of reference. Studies of this type provide important new insights into the nature of the interactions between the molecules and the superfluid solvent.