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 Title: CO2 dimer: four intermolecular modes observed via infrared combination bands Author(s): McKellar, Bob Contributor(s): Moazzen-Ahmadi, Nasser; Rezaei, Mojtaba; Dehghany, Mehdi; Norooz Oliaee, Jalal Subject(s): Clusters/Complexes 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$_{2}$ \nub{3} region ($\sim$2350 \wn), observed using tunable infrared lasers and a pulsed slit-jet supersonic expansion. The previous combination band at 2382.2 \wn was simple to assign [1]. A much more complicated band ($\sim$2370 \wn) turns out to involve $two$ upper states, one at 2369.0 \wn ($B_{u}$ symmetry), and the other at 2370.0 \wn ($A_{u}$). The spectrum can be nicely fit by including the Coriolis interactions between these states. Another complicated band around 2443 \wn 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 \wn band is the combination of the forbidden $A_{g}$ intramolecular fundamental (probably [1] at about 2346.76 \wn) and the intermolecular geared bend ($B_{u}$). The 2370.0 \wn band is the combination of the same $A_{g}$ fundamental and the intermolecular torsion ($A_{u}$). This gives about 22.3 and 23.2 \wn for the geared bend and torsion. The previous 2382.2 \wn band [1] is the allowed $B_{u}$ fundamental (2350.771 \wn) plus two quanta of the geared bend ($B_{u}$), giving 31.509 \wn for this overtone. The highly perturbed 2442.7 \wn band is the $B_{u}$ fundamental plus the antigeared bend ($A_{g}$), giving about 91.9 \wn for the antigeared bend. Finally, the perturbed 2442.1 \wn 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 \wn (antigeared bend), in good agreement with experiment. \vskip 0.2 truecm [1] M. Dehghany, A.R.W. McKellar, Mahin Afshari, and N. Moazzen-Ahmadi, \textit{Mol. Phys.} \textbf{108}, 2195 (2010). [2] X.-G. Wang, T. Carrington, Jr., and R. Dawes, private communication. Issue Date: 2016-06-22 Publisher: International Symposium on Molecular Spectroscopy Genre: Conference Paper/Presentation Type: Text Language: En URI: http://hdl.handle.net/2142/91235 Rights Information: Copyright 2016 by the authors Date Available in IDEALS: 2017-01-26
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