Direct measurements of line mixing, line broadening, and translational line shape in the nu(1) + nu(2) Q-branch of pure carbon dioxide

An infrared difference frequency spectrometer was improved such that the baseline spectra were constant to within 1 part in 1000 over a 1.3cm -1 scan. Over a 0.5 second integration time, transmission through the gas cell could be measured with a frequency resolution of 1.5MHz and a signal-to-noise ratio in excess of 2000:1. Using this spectrometer, line mixing and high resolution line shape effects have been directly measured for the nu 1 + nu2 band of CO2.; At pressures lower than 11kPa, the spectra were analysed within the Rosenkranz line mixing approximation. Translational effects were encompassed using the hard collision model of Nelkin and Ghatak (1964), modified to include an asymmetry component due to line mixing. For each line between Q(2) and Q(30), the Rosenkranz first order fine mixing coefficients, broadening coefficients, and narrowing parameters were measured. The fine mixing coefficients were found to be qualitatively consistent with, but significantly different from, the mixing coefficients calculated by Strow et al . (1994) using an Exponential Power Gap (EPG) law. The broadening coefficients, estimated to be accurate to within 0.5% on average, were found to be consistent with the P- and R-branch broadening measurements made by other groups. The narrowing parameters were found to be strongly non-linear with pressure, a symptom of speed-dependent fine shape effects unaccounted for by the line shape model used.; In addition to the line parameters, the band shift and band strength were measured to 5% and 0.5% accuracy respectively. No other shift measurements of this band have been reported for comparison. However, the band strength was found to be 3% lower than the currently used value of Hitran96 and 2% higher than other recent measurements of this band.; To analyse spectra at pressures greater than 11kPa, an empirical speed-dependent profile was formed from the sum of two hard collision profiles. Only with this profile could spectra up to 72kPa be fitted within error. The spectra displayed strong non-linear line mixing effects. In particular, intensity transfer between fines was directly measured and found to agree qualitatively with that predicted by an EPG scaling law calculation.