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.