Fundamental and Torsional Combination Bands of Two Isomers of the OCS–CO₂ Complex in the CO₂ ν₃ Region

Spectra of two isomers of the weakly bound complex OCS–CO₂ are observed in the region of the CO₂ ν₃ fundamental vibration (∼2349 cm^–1), using an infrared tunable diode laser to probe a pulsed supersonic slit-jet expansion. Two bands are measured and analyzed for each isomer, the fundamental asymmetric stretch of the CO₂ component and a combination band involving this fundamental plus the intermolecular out-of-plane torsional mode. For one isomer, the corresponding torsional combination band is also detected in the OCS ν₁ stretching region (∼2060 cm^–1). The resulting torsional frequencies are found to be 18.8 and 15.9 cm^–1 for isomers a and b of OCS–CO₂, respectively. This may be the first time that such a combination band is observed for a higher-energy isomer of a weakly bound complex.