15min:

Y. I. BARANOV, W. J. LAFFERTY AND G. T. FRASER, Optical Technology Division, NIST, Gaithersburg, MD 20899; A. A. VIGASIN, General Physics Institute, Russian Academy of Sciences, Vavilova 38, Moscow, 119991, Russia.

Two collision-induced (CIA) IR bands of CO₂ are observed in the region of the Raman allowed ₁- 2 ₂ Fermi-dyad monomer bands in the 7 µm region. These bands consist of a featureless CIA component upon which are superimposed very distinctive CO₂ dimer bands. The original observation of band structure in these bands was made by Welsh et al. and was interpreted by Mannik et al. to be the P-, Q- and R-branches of a T-shaped dimer. However, molecular beam studies 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 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 Å~ considerably larger than the molecular beam value of 3.3986 Å. 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 µ_a/µ_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 ^pP- and ^rR-branch lines. Comparison with observed spectra is good but not perfect and departures from the model will be discussed.