Infrared Spectral Lineshapes of Water Vapor and Carbon Dioxide.

Spectral lineshapes are investigated for pressure broadening conditions where the standard treatments fail. Emphasis is placed on the physical mechanisms responsible for the lineshapes, the overall accuracy of the results, and applicability to line-by-line radiative transfer codes. A phenomenological lineshape which includes the transfer of intensity between neighboring transitions (line -mixing) and includes duration of collision effects is developed for CO_2. Parameters representing the magnitude of line-mixing and the mean collision time are adjusted to obtain agreement with measured transmission spectra throughout and beyond the Sigma>=ts Sigma nu_3 R-branch at 4.3mum. Assuming a vibration independent collision time, the model is applied to the 15 μm region where perpendicular bands are dominant. For both regions, line-mixing is responsible for the majority of the observed sub-Lorentzian behavior. Good agreement with data is obtained for all pressures, temperatures, and perturbers. Another version of the model having a simpler form for the duration-of-collision effect is optimized for use directly at the nu_3 R-branch band-head, an important region for remote sensing applications. An experimental study of the "continuum" absorption within the nu_2 band of H _2O is presented. High and low optical depth self-, N_2-, and O _2-broadened transmission spectra were recorded using a Fourier transform spectrometer and long path absorption cell. Analysis of the continuum in the microwindows between lines shows general agreement with the laboratory results of Burch et al. and with recent atmospheric measurements. N _2-broadening results agree best with the CKDv0 model of Clough et al. at band center and with CKDv2.1 in the band wings. N_2 -broadened widths of over 150 H_sp{2 }{16}O transitions are measured in order to better characterize the H_2O lineshape. Our widths are systematically ~ 10% greater than values reported by Toth for HO _sp{2}{18}O, suggesting an isotopic dependence. Direct least-squares fits to the observed spectra were performed to determine the lineshape from near to far-wing. Results clearly show that the intermediate N_2-broadened lineshape is extremely super-Lorentz, suggesting that dispersion and/or inductive forces are important for medium range intermolecular separations.