Co2 Dimer: Four Intermolecular Modes Observed via Infrared Combination Bands

Abstract

Study of the carbon dioxide dimer has a long history, but there is only one previous observation of an intermolecular vibration [1]. Here we analyze four new combination bands of (CO 2) 2 in the CO 2 ν 3 region (∼2350 cm −1), observed using tunable infrared lasers and a pulsed slit-jet supersonic expansion. The previous combination band at 2382.2 cm −1 was simple to assign [1]. A much more complicated band (∼2370 cm −1) turns out to involve two upper states, one at 2369.0 cm −1 (B u symmetry), and the other at 2370.0 cm −1 (A u). The spectrum can be nicely fit by including the Coriolis interactions between these states. Another complicated band around 2443 cm −1 also involves two nearby upper states which are highly perturbed in so-far unexplained ways (possibly related to tunneling shifts). With the help of new ab initio calculations [2], we assign the results as follows. The 2369.0 cm −1 band is the combination of the forbidden A g intramolecular fundamental (probably [1] at about 2346.76 cm −1) and the intermolecular geared bend (B u). The 2370.0 cm −1 band is the combination of the same A g fundamental and the intermolecular torsion (A u). This gives about 22.3 and 23.2 cm −1 for the geared bend and torsion. The previous 2382.2 cm −1 band [1] is the allowed B u fundamental (2350.771 cm −1) plus two quanta of the geared bend (B u), giving 31.509 cm −1 for this overtone. The highly perturbed 2442.7 cm −1 band is the B u fundamental plus the antigeared bend (A g), giving about 91.9 cm −1 for the antigeared bend. Finally, the perturbed 2442.1 cm −1 band is due to an unknown combination of modes which gains intensity from the antigeared bend by a Fermi-type interaction. Calculated values [2] are: 20.64 (geared bend), 24.44 (torsion), 32.34 (geared bend overtone), and 92.30 cm −1 (antigeared bend), in good agreement with experiment.