This paper aims at examination of the (CO2)2 spectral absorption profile retrieved from the room temperature CO2 collision-induced absorption (CIA) spectrum in the region of the ν1, 2ν2 Fermi doublet. The assignment of (CO2)2 vibrations is discussed, paying due attention to high-resolution CARS observations in the regions of νlow and νup. A variational solution of the anharmonic vibrational problem for the (CO2)2 dimer was obtained using an extended basis set consisting of harmonic and Morse oscillator wave functions. This allowed for a satisfactory description of the vibrational dimer ν1, 2ν2 spectrum from first principles. In particular, we have succeeded in proving that the diffuse absorption band seen in the trough between the major Fermi-coupled CIA bands belongs to combinations of CO2 bending vibrations in a dimer.
High-resolution CARS spectroscopy has been employed to study the formation of small carbon dioxide clusters in a supersonic jet near the lower frequency component of the ν1/2ν2 Fermi dyad at 1285.41 cm−1. A pronounced structure was found at 1284.73 cm−1, redshifted by 0.68 cm−1 from the monomer Q-branch. Analysis of the complete set of experimental data and comparison with kinetic calculations indicate that the new feature must be attributed to the CO2 dimer.
Typical CARS spectra taken at high resolution in the regions of νlow and νup (14, 15, 23). Neat CO2 gas was expanded at a stagnation pressure of 1.5 bar through a ω = 90-μm-wide and l=1-mm-long slit nozzle. Both spectra were recorded at a relative distance of z/w = 4 from the nozzle exit.
14. A. A. Vigasin, A. A. Ilyukhin, L. Ramonat, V. V. Smirnov, O. M. Stelmakh, and F. Huisken, Khim. Fiz. 15, 88–95 (1996) [in Russian]. 15. F. Huisken, L. Ramonat, J. Santos, V. V. Smirnov, O. M. Stelmakh, and A. A. Vigasin, J. Mol. Struct. 47, 410–411 (1997). 23. L. Ramonat, Ph.D. Thesis, University of Gottingen, 1997.
