Observation of the `missing' polar OCS dimer.

The lowest energy, non-polar isomer of (OCS)$_{2}$ has long been known from infrared spectroscopy, while the polar form has only been deduced from qualitative beam ``refocusing'' experiment. We have observed a new infrared band at 2069.3 cm$^{-1}$ and assigned to the long-anticipated polar isomer of OCS dimer, helping to explain apparent discrepancies among earlier studies. Reported data which have been confirmed by direct observation of the rotational spectrum of polar OCS dimer, should motivate new theoretical work on the energies of OCS dimer isomers and interconversion energy barriers. OCS dimer is now one of the rare weakly bound clusters to have more than one isomer observed in high resolution spectroscopy.