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• Broadband, high-resolution dual frequency comb absorption spectroscopy of pure CO 2 between 6800 – 7000 cm-1 at temperatures up to 981 K. • Temperature-dependent line shape parameters measured for more than 110 transitions in the 00031←00001 and 01131←01101 bands using multispectrum analysis with both a Voigt and speed-dependent Voigt profile. • Comparison to calculated line shape parameters, including the updated parameter set used in the HITRAN2020 database. • Empirical models for the broadening coefficient, pressure shift coefficient, and their temperature dependence validated for high-temperature transitions up to | m |≈90. We present the first systematic study of self-broadened CO 2 line shape parameters in the 00031 ← 00001 and 01131 ← 01101 bands at high temperature. We report temperature-dependent broadening coefficients and pressure shift coefficients for more than 40 transitions in the 00031 ← 00001 band, and 75 transitions in the 01131 ← 01101 hot band. The majority of these parameters have never been measured at high temperature. The new parameters are determined from 12 spectra measured with a broadband, high-resolution dual frequency comb absorption spectrometer in a high-temperature gas cell from room temperature to 980 K. We fit the measured spectra using a multispectrum fitting approach to derive line shape parameters with both a Voigt and speed-dependent Voigt profile. We compare the measured parameters to calculated line shape parameters as well as to the new parameter set developed for the HITRAN2020 database. The new measurements demonstrate strong agreement with HITRAN2020 (within 1% in some cases), and provide new measurements of the temperature dependence of the broadening and pressure shift coefficient that will improve the accuracy of the database at high temperature. We extend our results to high | m | transitions using a set of empirical models for the broadening and pressure shift coefficients, which we fit to our data and validate for high-temperature transitions up to | m | = 98. We use these models to construct a set of database files that can be used to supplement the HITRAN database for self-broadened CO 2 in the 6800 – 7000 cm-1 region. These new parameters significantly improve the accuracy of absorption models for high-temperature CO 2 in this frequency range, and also provide useful data to test and improve methods used to calculate CO 2 line shape parameters. [ABSTRACT FROM AUTHOR]

• Broadband, high-resolution dual frequency comb absorption spectroscopy of CO 2 between 6800 and 7000 cm−1 in high-pressure, high-temperature conditions up to 25 bar and 977 K. • Temperature-dependent models for line mixing based on energy-corrected sudden and modified exponential gap scaling laws. • Comparison of line mixing models to broadband phase spectra measured at pressures up to 25 bar near room temperature. We perform dual frequency comb laser absorption spectroscopy of CO 2 at high pressure and temperature, and use the spectra to test and improve CO 2 line mixing models and their temperature dependence. The dual-comb spectrometer spans 6800–7000 cm−1 with 0.0066 cm−1 point spacing, and is coupled to a specialized, high-pressure, high-temperature gas cell providing conditions up to 977 K and 25 bar. We compare our measurements to spectra calculated using an advanced line mixing model for pure CO 2 based on the energy-corrected sudden (ECS) approximation. We determine a new set of temperature-dependent ECS model parameters, and show that the new parameters significantly improve the accuracy of the ECS line mixing model over a temperature range spanning 298–977 K. We also compare the measured spectra to a simpler line mixing model developed using the modified exponential gap (MEG) scaling law, and report the temperature and pressure dependence of the MEG model parameters required to scale the model across wide ranges of conditions. Finally, we report high-pressure, ambient-temperature measurements of CO 2 spectra in both amplitude (absorption) and phase using the dual-comb spectrometer. We use these measured absorption and phase spectra to assess the performance of the ECS and MEG line mixing models at high densities near room temperature. To the best of our knowledge, these results represent the first study of line mixing using phase spectroscopy. Overall, the results of this study significantly expand line mixing models for CO 2 at high pressure and temperature and improve the accuracy and availability of absorption models for harsh conditions encountered in laser-based sensing and planetary science. [ABSTRACT FROM AUTHOR]

• Technique to broaden absorption cross-section datasets to higher pressure conditions without knowledge of individual line parameters. • Based on scaling the exponential decay envelope of the molecular free induction decay formed from the inverse fourier transform of an absorption cross-section at a particular pressure. • Validated for three complex molecules using published absorption cross section data at different pressure conditions. The accuracy of absorption-based measurement is directly linked to the fidelity of the pressure- and temperature-dependent reference absorption cross-sections used for fitting and data interpretation. For large, complex molecules or species for which line-by-line absorption databases do not exist, cross-sections are often measured in the laboratory at a particular set of known temperature and pressure conditions and published for use as reference spectra. For subsequent simulations or measurements at conditions between or outside of the reference conditions, no exact reference cross-sections exist. We propose a technique which enables the scaling of reference absorption cross-sections to different pressures. Pressure influences the shape of the cross-section through collisional broadening. We modify the collisional broadening of the cross-section by performing an inverse Fourier transform on the dataset to view the free induction decay of the molecule, and then computationally adjusting the decay rate of the FID. This effectively pressure scales the broadening parameters of the multitude of contributing molecular resonances. The technique is validated here against reference absorption cross-sections, and can yield residuals between modified and measured reference spectra below one percent. The approach is simple to apply, and allows users to generate reference cross-sections at any pressure above the pressure condition of the reference dataset. It therefore also significantly reduces the need to measure reference absorption cross-sections at many pressures. [ABSTRACT FROM AUTHOR]

• Design of a highly uniform, long path length, optically accessible gas cell for laboratory absorption spectroscopy at high-pressure and -temperature conditions. • Complete characterization of the gas cell temperature uniformity up to 1000 K and 50 bar demonstrates that temperature deviations remain less than 4.5% across the 45.82 cm absorbing path length at all conditions. • Broadband, high resolution spectroscopy of the CO2 3v3band at pressures between 0.2 and 20 bar and temperatures up to 1000 K using a dual frequency comb absorption spectrometer. • Comparison to a HITRAN2016-based absorption model demonstrates the need to improve line shape parameters for high temperatures as well as evidence of non-Lorentzian effects (line mixing) in the high-pressure spectra. Accurate absorption models for gases at high pressure and temperature support advanced optical combustion diagnostics and aid in the study of harsh planetary atmospheres. Developing and validating absorption models for these applications requires recreating the extreme temperature and pressure conditions of these environments in static, uniform, well-known conditions in the laboratory. Here, we present the design of a new gas cell to enable reference-quality absorption spectroscopy at high pressure and temperature. The design centers on a carefully controlled quartz sample cell housed at the core of a pressurized ceramic furnace. The half-meter sample cell is relatively long compared to past high-pressure and -temperature absorption cells, and is surrounded by a molybdenum heat spreader that enables high temperature uniformity over the full length of the absorbing gas. We measure the temperature distribution of the sample gas using in situ thermocouples, and fully characterize the temperature uniformity across a full matrix of temperatures and pressures up to 1000 K and 50 bar. The results demonstrate that the new design enables highly uniform and precisely known temperature and pressure conditions across the full absorbing path length. Uniquely, we test the new gas cell with a broadband (~2500 cm−1), high-resolution (0.0066 cm−1) dual frequency comb spectrometer that enables highly resolved absorption spectroscopy across a wide range of temperature and pressure conditions. With this carefully characterized system, we measure the spectrum of CO 2 between 6800 and 7000 cm−1 at pressures between 0.2 and 20 bar, and temperatures up to 1000 K. The measurements reveal discrepancies from spectra predicted by the HITRAN2016 database with a Voigt line shape at both low- and high-pressure conditions. These results motivate future work to expand absorption models and databases to accurately model high-pressure and -temperature spectra in combustion and planetary science research. [ABSTRACT FROM AUTHOR]