  Long-range intermolecular forces are able to steer polar molecules submerged
in superfluid helium nanodroplets into highly polar metastable configurations.
We demonstrate that the presence of such special structures can be identified,
in a direct and determinative way, by electrostatic deflection of the doped
nanodroplet beam. The measurement also establishes the structures' electric
dipole moments. In consequence, the introduced approach is complementary to
spectroscopic studies of low-temperature molecular assembly reactions. It is
enabled by the fact that within the cold superfluid matrix the molecular
dipoles become nearly completely oriented by the applied electric field. As a
result, the massive (tens of thousands of helium atoms) nanodroplets undergo
significant deflections. The method is illustrated here by an application to
dimers and trimers of dimethyl sulfoxide (DMSO) molecules. We interpret the
experimental results with ab initio theory, mapping the potential energy
surface of DMSO complexes and simulating their low temperature aggregation
dynamics.
