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A spectrum of the carbon dioxide trimer van der Waals species has been recorded near 3614 cm−1 at sub-Doppler resolution using an optothermal (bolometer-detected) molecular-beam color-center laser spectrometer. A planar, cyclic structure with C3h symmetry has been determined for the complex with a carbon–carbon separation of 4.0382(3) Å. The observed perpendicular band, corresponding to an in-plane E′-symmetry vibration of the trimer, has been attributed to a localized excitation of the 2ν02 +ν3 combination mode of a CO2 subunit by virtue of its small blue shift (∼0.98 cm−1) from that of the isolated monomer.
Fraser et al. (Sat,) studied this question.