ON THE ORIGIN OF THE BAND STRUCTURE OBSERVED IN THE COLLISION-INDUCED ABSORPTION BANDS OF $CO_{2}$

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 Title: ON THE ORIGIN OF THE BAND STRUCTURE OBSERVED IN THE COLLISION-INDUCED ABSORPTION BANDS OF $CO_{2}$ Creators: Baranov, Y. I.; Lafferty, W. J.; Fraser, G. T.; Vigasin, A. A. Issue Date: 2003 Abstract: Two collision-induced (CIA) IR bands of $CO_{2}$ are observed in the region of the Raman allowed $\nu_{1}- 2\nu_{2}$ Fermi-dyad monomer bands in the $7 \mu$ m region. These bands consist of a featureless CIA component upon which are superimposed very distinctive $CO_{2}$ dimer bands. The original observation of band structure in these bands was made by Welsh $et al.^{a}$ and was interpreted by $Mannik et al.^{b}$ to be the P-, Q- and R-branches of a T-shaped dimer. However, molecular beam $studies^{c}$ have subsequently shown that the dimer bands consist of nearly equally intense a-type and b-type transitions and that the dimer structure is a slipped-parallel arrangement with $C_{2h}$ symmetry. Recently Vigasin and $Baranov^{d}$ have modeled the dimer profile observed in room temperature CIA spectra using a symmetric-rotor model which leads to a derived C-C separation of $4.46 \AA$ considerably larger than the molecular beam value of $3.3986 \AA$. In this report, we suggest an alternative explanation for the dimer band profile observed. We have modeled the bands using a Watson asymmetric-rotor Hamiltonian and the rotational constants derived in a molecular beam study. We have varied only the upper-state A rotational constant and the $\mu_{a}/\mu_{b}$ ratio. In this model, the band structure is not due to P- Q, and R-branches but rather to a central sharp a-type Q-branch with broad b-type Q-branches on either side of the band center. All the Q-branch transitions sit on a pedestal of unresolved $^{p}P-$ and $^{r}R$-branch lines. Comparison with observed spectra is good but not perfect and departures from the model will be discussed. URI: http://hdl.handle.net/1811/20747 Other Identifiers: 2003-FB-06