Signatures of large amplitude motion in a weakly bound complex: High‐resolution IR spectroscopy and quantum calculations for HeCO2

The infrared spectrum of the HeCO2 van der Waals molecule is recorded in the region of the CO2 ν3 asymmetric stretch via direct absorption of a tunable Pb–salt diode laser. HeCO2 is formed in a slit jet supersonic expansion; the slit valve and the stagnation gas must be precooled to −35 °C before substantial formation of the complex is observed. Sixty‐six rovibrational transitions are recorded by exciting the ν3 asymmetric stretch of the CO2 monomer within the complex. Forty‐three of these transitions can be assigned using internally consistent combination differences as a b‐type band of a T‐shaped asymmetric rotor. There are several indications that large amplitude motion is significant in HeCO2, including the poor quality of the fit to an asymmetric rotor model and the large positive inertial defects of Δ=8.54 and 10.98 uA2 in the ground and excited states, respectively. However, a hindered rotor analysis based on these inertial defects demonstrates that the CO2 motion within the complex is far from the...