High-pressure measurements of CO₂ absorption near 2.7 μm: Line mixing and finite duration collision effects

Room-temperature, high-pressure (1-30 atm) measurements of CO₂ absorption are carried out near 2.7 μm to study line mixing and finite duration collision effects on transitions in the ν₁+ν₃ and 2ν₂+ν₃ vibrational bands. Two distributed feedback diode lasers are used to measure CO₂ transitions near 3631-3635 cm^-1 and 3644-3646 cm^-1, and an FTIR spectrometer covers the entire ν₁+ν₃ and 2ν₂+ν₃ bands from 3500 to 3800 cm^-1. The experiments are carried out in CO₂-air and CO₂-Ar mixtures to observe the non-ideal effects under the influence of different perturbers. Measurements are compared with simulations using the Voigt line shape to analyze the deviation from the Lorentzian behavior with increasing gas density, and show significant deviation from this model at high gas densities. Line shape models using empirical corrections or dynamically based scaling laws are evaluated by comparison to the measured high-density spectra. Although none of the models is able to predict the measured spectra accurately, the line mixing model of Niro et al. [24] does an overall good job but overestimates the band centers by about 4-9%. In light of these observations, challenges of developing a CO₂ sensor for high-pressure combustion applications are discussed.