INTERBRANCH LINE-MIXING IN CO2 (10(0)1)COMBINATION AND (02(0)1)COMBINATION BANDS

Absorption spectra from a mixture of 320 ppm CO2 in synthetic air (79% N-2, 21% O-2) were collected in the region from 3500 cm(-1) to 4000 c m(-1) under conditions in the range of 100-1000 atm and 295-900 K. At 295 K, both bands of the (10 degrees 1), (02 degrees 1) Fermi dyad sho w the collapse of P and R branches into a single nearly Lorentzian spe ctral feature as a result of interbranch line-mixing. At elevated temp eratures, the presence of interbranch mixing is also clearly evident a s is the presence of several hot bands. The experimental data are mode led using two methods for simulating line-mixed spectra; first, the us ual line-by-line approach which relies on the binary impact approximat ion, and second, a simple band-averaged model proposed by Hartmann and L'Haridon [J. Chem. Phys. 103, 6467 (1995)]. The energy corrected sud den (ECS) approximation is used to generate the relaxation matrix in t he first approach. Comparison with the measurement shows that the ECS method does not fit the high density data satisfactorily when adjustab le parameters from the literature are used; the level of interbranch m ixing must be decreased by about a factor of 2 relative to intrabranch mixing and at least 5% dephasing must be added to the ECS matrix. Wit h these changes, the room temperature data are modeled satisfactorily, but significant discrepancies are still present in the high temperatu re spectra. On the other hand, the simpler band-averaged model does pr ovide a reasonable estimate of the spectra for all temperatures when b est fit values are used for mixing and broadening, but the low density data are not reproduced as well as with the ECS model. Data from high pressure absorption measurements in a 1% NO in N-2 mixture as well as a 0.5% CH4 in N-2 mixture are also presented without analysis, showin g the effects of interbranch line-mixing in these spectra. (C) 1997 Am erican Institute of Physics.