Five intermolecular vibrations of the CO dimer observed via infrared combination bands.

The weakly bound van der Waals dimer (CO) has long been of considerable theoretical and experimental interest. Here, we study its low frequency intermolecular vibrations by means of combination bands in the region of the CO monomer ν fundamental (≈2350 cm), which are observed using a tunable infrared laser to probe a pulsed supersonic slit jet expansion. With the help of a recent high level ab initio calculation by Wang, Carrington, and Dawes, four intermolecular frequencies are assigned: the in-plane disrotatory bend (22.26 cm); the out-of-plane torsion (23.24 cm); twice the disrotatory bend (31.51 cm); and the in-plane conrotatory bend (92.25 cm). The disrotatory bend and torsion, separated by only 0.98 cm, are strongly mixed by Coriolis interactions. The disrotatory bend overtone is well behaved, but the conrotatory bend is highly perturbed and could not be well fitted. The latter perturbations could be due to tunneling effects, which have not previously been observed experimentally for CO dimer. A fifth combination band, located 1.3 cm below the conrotatory bend, remains unassigned.