Observation of the elusive "oxygen-in" OCS dimer.

The carbonyl sulfide (OCS) dimer serves as a prototype system for studying intermolecular forces between nonsymmetrical linear polyatomic molecules. Here, we performed a laser spectroscopic investigation of OCS dimers embedded in helium nanodroplets and found rovibrational bands corresponding to the non-polar "sulfur-in" and parallel polar dimers that have been extensively characterized in the gas phase, as well as a new non-polar "oxygen-in" dimer that has long been predicted by theory. Frequency alternations in the rotational branches along with the absence of a Stark effect provided important clues as to its assignment.