B. Khalil

Temperature, pressure, and perturber dependencies of line-mixing effects in CO2 infrared spectra. I. Σ←Π Q branches

Experimental and theoretical results on the influence of line mixing on the shape of infrared CO2 Q branches of importance for atmospheric applications are presented. Two Q branches of Σ←Π symmetry, which lie near 618 and 720 cm−1 and belong to the 1000II←0110I and 1000I←0110I bands, have been studied for many conditions of temperature (200–300 K), total pressure (0.5–10 atm), and mixture (with He, Ar, O2, and N2). The theoretical approach used is based on the Energy Corrected Sudden approximation; its parameters have been deduced from both line-broadening data and measured absorption by the Q branches. Comparisons between experimental and computed spectra demonstrate the quality of the model, regardless of the conditions. Detailed analysis of the influences of the Q-lines spectral spacing, temperature, total pressure, and collision partner are presented. They show that significantly larger line-mixing effects are obtained when CO2–He is considered with respect to CO2–(Ar,O2,N2). This is analyzed in terms...

Q-branch shapes of CO2 spectrum in 15 μm region: Experiment

Abstract The shapes of Q -branches in CO 2 spectra in the 580–850 cm −1 region for pure gas and mixtures CO 2 He and CO 2 Ar have been studied at resolutions up to 0.002cm −1 . The branch broadening coefficients were measured in the pressure range from a few torr to 50 atm. Those for 11 1 02-02 2 01 Π-Δ transition are different for pressures below and above 5 atm. At higher pressures the branch broadening coefficients are similar for all Q -branches. The observed Q -branch shape transformation is explained by line mixing effects.

An optimized line by line code for plume signature calculations II. Comparisons with measurements

Abstract Measurements of transmission spectra in the infrared have been made in order to test the quality of the theoretical and numerical approach developed in paper I. Two different laboratory experiments have been built which try to simulate optical paths representative of those involved by plume signature. The first, based on a 50 m long White cell and a grating spectrometer, provides experimental results for optical paths of up to about 1 km . These data, representative of the atmospheric paths involved by the considered signature applications, enable a check of the model used for far line wings. In the second set-up, a Fourier transform spectrometer is used with two adjacent cells, which are at high and low temperature, respectively. This configuration schematizes the hot+cold gases that are involved in plume signature. Comparisons between measured and computed values demonstrate the quality of the model and data used. Only small discrepancies remain which are due to imperfections of spectroscopic databases and the neglect of line-mixing effects.

SIMPLE MODELLING OF Q-BRANCH ABSORPTION. I: THEORETICAL MODEL AND APPLICATION TO CO2 AND N2O

Abstract A simple theoretical approach of Q-branch absorption is developed. It is based on the classical approximation of the rotational distribution and a Strong-Collision-type modelling of line-coupling coefficients. The Q-branch absorption is then represented by a very simple analytical expression which depends on six average parameters; pressure and wave-number dependences are included in the model so that the parameters only depend on the molecular system (active molecule + perturber), band, and temperature. Tests show that, provided effective parameters are used, our model enables very satisfactory predictions of the pressure, temperature, and wavenumber dependences of Q-branch absorption. These effective parameters can be deduced from experimental spectra and no previous knowledge of neither the spectroscopic nor collision parameters is required. This makes the present approach suitable for species (most of those of atmospheric interest) for which no alternative and more accurate approach is yet available.

Argon-broadened CO2 linewidths at high J values

Abstract We present experimental collision-broadening coefficients at high J values obtained in the ν 3 band of CO 2 pressurised by argon. They were determined by Fourier transform spectroscopy at room temperature for pressures up to 2 atm. We review recent experimental and theoretical data and compare them with calculations performed by using the energy corrected sudden (ECS) approximation. The present extended measurements allow a better determination of the basic parameters which are required to model the line coupling induced by collision within this formalism.

Calculations and measurements of N2O absorption at high temperature in the 4.5 μm region

Abstract An approximate N 2 O spectroscopic database suitable for high temperature and medium resolution applications has been created in the 4.5 μm region. Intensities of 14 N 2 16 O hot bands have been extrapolated pragmatically from the v 3 band intensity and energies of vibrational levels have been computed by diagonalization of the effective Hamiltonian. The new parameters have been merged with the data available in the HITRAN database and in the recent experimental work of Toth. The entire list has then been used to generate individual line parameters. Pure N 2 O spectra have been recorded with a Fourier Transform spectrometer up to 900 K and with 1 cm −1 resolution. A good agreement between these spectra and line-by-line calculations using the new database is obtained while the use of HITRAN greatly underestimates absorption at high temperature.

Line mixing and line broadening in CO2 bands perturbed by helium at 193 K

Abstract The ν2 and 3ν3 bands of CO2 in helium baths at 193 K have studied with a Fourier transform interferometer. The behavior of the band shapes has been explored at moderate densities. The energy corrected sudden (ECS) approximation is used to model the relaxation matrix in order to account for line mixing effects. The basis cross-sections were calculated with the simple power law (P). Computed spectra are in good agreement with the observed ones. Measured broadening coefficients are also comparable with the ones derived from the ECS-P model.

Line broadening and band profile of CO2 in helium at 193 K

In order to investigate the temperature dependence of broadening parameters and band profiles we have studied the v2 and 3v3 bands of carbon-dioxide perturbed by He at 193 K. We used the energy corrected sudden (ECS) scaling law associated with the power fitting law. Experiments were carried out with a Fourier transform spectrometer.