The weak infrared spectrum of CO 2-Ar corresponding to the (v 1 , v l2 2 , v 3) = (01 1 1) ← (01 1 0) hot band of CO 2 is detected in the region of the carbon dioxide ν 3 fundamental vibration (∼2340 cm −1), using a tunable OPO to probe a pulsed supersonic slit jet expansion. While this method was previously thought to cool clusters to the lowest rotational states of the ground vibrational state, here we show that under suitable jet expansion conditions, sufficient population remains in the first excited bending mode of CO 2 (1-2%) to enable observation of vibrationally hot CO 2-Ar, and thus to investigate the symmetry breaking of the intramolecular bending mode of CO 2 in the presence of Ar. The bending mode of CO 2 monomer splits into an in-plane and an out-of-plane mode, strongly linked by a Coriolis interaction. Analysis of the spectrum yields a direct measurement of the in-plane / out-of-plane splitting measured to be 0.8770 cm −1. This aspect of intramolecular interactions has received little previous experimental and theoretical consideration. Therefore, we provide an additional avenue by which to study the intramolecular dynamics of this simplest dimer in its bending modes. Similar results are obtained for CO 2-Ne.

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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.

Only a few weakly-bound complexes containing the O 2 molecule have been characterized by high-resolution spectroscopy, notably N 2 O-O 2 [1] and HF-O 2 [2]. This neglect is no doubt due in part to the complications added by the oxygen unpaired electron spin. Here we report an extensive infrared spectrum of CO-O 2 , as observed in the CO fundamental band region (∼2150 cm −1 ) using a tunable quantum cascade laser to probe a pulsed supersonic jet expansion. The derived energy level pattern consists of 'stacks' characterized by K, the projection of the total angular momentum on the intermolecular axis. Five such stacks are observed in the ground vibrational state, and ten in the excited state, v(CO) = 1. They are divided into two groups, with no observed transitions between groups, and we believe these groups correlate with the two lowest rotational states of O 2 , namely (N , J) = (1, 0) and (1, 2). In many ways, the spectrum and energy levels are similar to those of CO-N 2 [3], and we use the same approach for analysis, simply fitting each stack with its own origin, B-value, and distortion constants. The rotational constant of the lowest stack in the ground state (with K = 0) implies an effective intermolecular separation of 3.82 Å, but this should be interpreted with caution since it ignores possible effects of electron spin.

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