To meet the challenges of high-resolution molecular spectroscopy, increasingly sophisticated spectroscopic techniques were developed. For a long time FTIR and laser-based spectroscopies were used for these studies. The recent development of dual-comb spectroscopy at high-resolution makes this technique a powerful tool for gas phase studies. We report on the use and characterization of the IRis-F1, a tabletop mid-infrared dual-comb spectrometer, in the newly developed step-sweep mode. The resolution of the wavenumber axis is increased by step-wise tuning (interleaving) and accurate measurement of the laser center wavelength and repetition frequency. Doppler limited measurements of N₂O and CH₄ reveal a wavenumber accuracy of 10⁻⁴ cm⁻¹ on the complete covered range of 50 cm⁻¹. Measured half-widths of absorption lines show no systematic broadening, indicating a negligible instrument response function. Finally, measurements of nitrogen pressure broadening coefficients in the ν₄ band of methane show that the dual-comb spectrometer in step-sweep mode is well adapted for measurements of precision spectroscopic data, in particular line shape parameters.

We apply laser absorption spectroscopy to design a diagnostic measuring bulk gas pressure from collisionally-broadened absorption lineshapes. This diagnostic targets atomic potassium with a measurement rate of 200 kHz. The diagnostic is intended to operate in hypersonic ground-based facilities, where atomic potassium is nascent in the freestream flow and where microsecond temporal resolution is often crucial. Recent studies have found atomic potassium in trace amounts in the freestream of hypersonic ground-based facilities, making it an attractive spectroscopic target. Potassium also has convenient spectroscopic transitions in the near-infrared – the D-line transitions (²S_1/2 → ²P_1/2 at 770.1 nm and ²S_1/2 → ²P_3/2 at 766.7 nm), which absorb strongly and are easily accessible with low-cost commercial lasers and optics. This line-of-sight laser-based diagnostic infers bulk gas pressure from the spectroscopic lineshape of the potassium D₂ transition, specifically the collisional linewidth parameter ∆ν_C. We apply empirical correlations to extract pressures from a Voigt fit of these lineshapes. These correlations depend on gas composition and temperature, which must be known. Lineshape parameters must also be corrected to account for power broadening effects, and hyperfine splitting is considered at low pressures. For verification, the diagnostic is deployed in a shock tube to generate the temperatures, pressures, and timescales relevant to freestream flows in hypersonic ground-based facilities. Since atomic potassium is not present in sufficient quantity for measurement in our shock tube, we implement a novel technique to uniformly seed potassium into the shock-heated gas. We achieve excellent signal-to-noise ratios and measure pressures in good agreement with expected values between 0.25-2 atmospheres.

When an observed profile of a spectral line is analyzed using an equally weighted least squares method, the noise property of the spectrum determines whether transmittance or absorbance spectrum is appropriate for the analysis. To verify this, we simulate transmittance spectra (TS) of Lorentz profiles with three simulation parameters of absorption strength, center frequency, and width and add either detector noise (DN) or source noise (SN) to the simulated TS. The TSs with DN or SN and absorbance spectra (AS), negative logarithms of them, are fitted to the Lorentz profile using least squares methods. Equally weighted fits of TS with DN and AS with SN, as statistic mathematics predicts, reproduce the noise magnitude and the parameters well and give the expected uncertainties close to the standard deviations of a thousand simulated spectra regardless of the absorption intensity and the noise magnitude. In contrast, equally weighted fits of TS with SN and AS with DN reproduce the simulation parameters but not the noise magnitude and do not predict the uncertainties of the parameters. Properly weighted fits of TS with SN and AS with DN reproduce the noise magnitude and give the expected uncertainties like those given from equally weighted fits of AS with SN and TS with DN but do not always reproduce the absorption strength and width.

Collisional energy transfer in volatized exospheric materials dominates the uncertainty of comet models that trace comae composition back to surface composition. Methods for ab initio and semi-empirical calculation of quantum-state dependent collisional efficiencies are typically benchmarked to pressure broadening experiments when available. Here we detail experimental efforts to determine collisional efficiencies for selected transitions of water at temperatures demonstrative of the comet environment and well below the water condensation temperature. The method utilizes a collisional cooling cell with water injected into a bath gas at the target temperature. THz radiation is passed twice through the cooled gas to record a transmission spectrum exhibiting the Lamb dip effect. The sub-Doppler feature is subject to collisional broadening at pressures commensurate with the partial pressure of water in the system. Data analysis involves simultaneous extraction of intensity and pressure broadening information. The method, results and comparisons to calculated values will be discussed.

Understanding collision-induced absorption (CIA) is a critical component to improving the O₂ spectroscopy for remote sensing applications. Traditionally in experimental spectra, CIA is defined as the remaining absorption after accounting for the baseline, Rayleigh scattering, and resonant absorption. This approach can present difficulties in systems, like the O₂ A-Band at 0.76 μm, where the CIA is relatively weak and is highly correlated with the line-mixing model. Theoretical constraints on the magnitude and shape of the CIA could aid in decoupling the resonant and broadband features ultimately leading to an improved spectroscopic model. The CIA model reported by Karman et al. [1] provides a theoretical basis for the CIA in the 1.27 μm and 0.76 μm O₂ bands [1]. In this work, we evaluate the theoretical model using cavity ring-down spectroscopy measurements collected at multiple temperatures in both the 1.27 μm [2-4] and 0.76 μm O₂ bands. In addition to a qualitative comparison between experiment and theory, this work explores parameterization of the CIA model reported by Karman et al. [1] for future inclusion in integrated multi-spectrum analyses incorporating advanced line shape models, line-mixing, and CIA. [1] Karman T, et al. Nature Chemistry. 2018;10:549-54. [2] Kassi S, et al. Journal of Geophysical Research: Atmospheres. 2021;126. [3] Mondelain D, et al. Journal of Geophysical Research: Atmospheres. 2019;124:414-23. [4] Fleurbaey H, et al. Journal of Quantitative Spectroscopy and Radiative Transfer. 2021;270.

Accurate knowledge of the absorption by a gas mixture of CO₂ and water is crucial for planetary sciences, as it allows for better modeling the atmospheres of rocky planets, e.g. improving our understanding of the early climate of Mars or why Venus and the Earth have evolved so differently. In addition to local monomer lines proportional to the density of each species, the absorption spectrum of such a gas mixture includes binary absorption features varying smoothly with frequency: self-continuum absorption proportional to the squared density, and "crossed" absorption involving both species and scaling as the density product ρ_CO2ρ_H2O. We used highly sensitive spectroscopy techniques (CRDS and OFCEAS) to measure the absorption by H₂O+CO₂ gas mixtures in several spectral regions situated in transparency windows where the monomer absorption of both species is weak (1.5-1.53 μm, 1.68-1.75 μm, 2.06 μm, 2.2-2.35 μm, 3.5 μm). For both water and CO₂, the monomer lines, modeled using HITRAN parameters, and the self-continuum absorption, calculated from literature values or measured in dedicated experiments, were subtracted from the measured absorption. The obtained "crossed absorption" coefficients are compared to the only available empirical model based on far wings of line shape profiles scaled by χ-factors.Fleurbaey H, Campargue A, Carreira Mendès Da Silva Y, Grilli R, Kassi S, Mondelain D. J Quant Spectrosc Radiat Transf 108119 (2022)n additional absorption peak centered at about 6000 cm⁻¹was attributed to a collision-induced simultaneous transition of H₂O and CO₂ through the ν₁ and ν₃ modes, respectively. The assignment was confirmed using humidified ¹³CO₂, where a similar band was observed about 68 cm⁻¹away corresponding to the isotopic spectral shift of the ν₃ band of CO₂. Classical molecular dynamics simulations (CMDS) of the considered collision-induced absorption were conducted and are found in good agreement with the experiment.Fleurbaey H, Mondelain D, Fakhardji W, Hartmann J-M, Campargue A. Submitted to J Quant Spectrosc Radiat Transfhtml:<hr /><h3>Footnotes: Fleurbaey H, Campargue A, Carreira Mendès Da Silva Y, Grilli R, Kassi S, Mondelain D. J Quant Spectrosc Radiat Transf 108119 (2022)A Fleurbaey H, Mondelain D, Fakhardji W, Hartmann J-M, Campargue A. Submitted to J Quant Spectrosc Radiat Transf

r0pt Figure Upcoming exoplanet infrared imaging will likely include carbon monoxide (CO) absorption from deeper, higher-pressure regions of larger Jupiter-like exoplanets, with compositions of majority hydrogen (H₂) and helium (He). However, there have been limited experimental CO spectroscopy studies in H₂ and He at elevated pressure conditions. We present quantitative, broadband absorbance measurements of the fundamental ro-vibrational band of CO between 1965 and 2235 cm⁻¹, in bath gases of nitrogen (N₂), He, and H₂. Then, we demonstrate a modeling approach that accurately reflects the effects of line mixing that we observe in the results, utilizing the modified exponential gap (MEG) law with a fitted inter-branch factor. The room-temperature static cell measurements were taken using a narrow-linewidth, broad-scan external-cavity quantum-cascade laser at pressures of 1535 atm. For CO in H₂ and He, minor adjustments to the MEG Law were necessary to reproduce the weaker J"-dependence of the broadening coefficients relative to that of CO in N₂. The resulting MEG line mixing model shows improved agreement with the measured spectra across different pressures and broadening partners. Further reduction of the residuals to within approximately 1% (CO/H₂, 35 atm) is shown through the fitting of MEG coefficients directly to measured spectra, resulting in relatively small adjustments to each of the coefficients.

We present temperature-dependent cross sections for trans-2-Butene (trans-2-C₄H₈: CH₃-CH=CH-CH₃) in the 7 - 15 μm region in support of remote sensing of Titan’s stratosphere. It is one of many C₄-hydrocarbons predicted to be in detectable abundances in Titan’s atmosphere by photochemical models, but no high-resolution spectroscopy is available in the public databases in the mid-infrared region, let alone at cold temperatures appropriate for Titan. We collected 28 pure and N₂-mixture spectra and their corresponding background spectra at temperatures between 160-297 K using a Fourier transform spectrometer (Bruker IFS-125HR) at the Jet Propulsion Laboratory at spectral resolutions between 0.0039 and 0.062 cm⁻¹, depending on sample pressures in consideration of line shape resolving power. We obtained transmission spectra by ratioing the sample spectra to their empty-cell spectra, from which several fundamental modes of vibration were identified and updated in comparison to their band centers reported in the literature. We defined two distinct spectral regions, each of which contains multiple vibrational bands and hot band features and we measured the temperature-dependent cross sections, and report their integrated cross sections as well. We performed a separate linearity test between the sample absorbance and optical burden for the spectra obtained at various sample pressures. No significant dependence on temperature was observed in the integrated cross sections, which validated our measurements and methodology. Our measured cross sections will provide critical laboratory input toward a search for trans-2-Butene in Titan stratosphere that may be captured in the Cassini/CIRS spectra. To facilitate this, we will report our final results to the public databases, such as HITRAN and GEISA.Government support acknowledged.html:<hr /><h3>Footnotes: Government support acknowledged.

r0pt Figure Potassium can be used as a convenient tracer species in combustion and hypersonic test facilities and is naturally present in trace amounts in the atmospheres of brown dwarfs, where the resonance doublet is highly detectable. Currently, there are no experimental data of potassium lineshape parameters at temperatures over 500 K and model predictions vary widely above 1000 K. We present measurements of collisional broadening and pressure shift parameters for the potassium D-lines, near 770 nm, with collisional partners of N₂, He, and H₂. Atomic potassium is generated in a shock tube by shock heating KCl salts at temperatures between 1100-1900 K, and line parameters are measured using rapid-scanning tunable diode laser absorption spectroscopy. The lineshape measurements were modeled as Voigt profiles and a fitting algorithm determined pressure shift and collisional full-width-at-half-maximum. The collisional broadening and pressure shift coefficients are given as temperature-dependent power-law relations for the partners of interest. The helium and hydrogen results agree with lower temperature experimental data, within 15-20%, and high-temperature theoretical predictions, within 10-30%. The nitrogen results, however, have larger discrepancies with existing data and simplified impact theory predictions. This may suggest the need for a more detailed model for the nitrogen collisional broadening of potassium. The presented correlations may be useful for the development of potassium-based sensing methods with application to combustion, hypersonics, and astrophysics.

One of the key findings regarding exoplanet science is that majority of the detected close-in planets from Kepler fall within the super-Earth/sub-Neptune regime 1-3.5 Earth Radii. Planet formation models of these systems suggest broad compositional diversity in this radius regime, with a high likelihood for large atmospheric metal content 100-1000xSolar. Our ability to unlock the mysteries of this new class of planet hinges on our ability to link the spectral observations to theoretical models, and then our ability to link those models to fundamental molecular and atomic opacities. However, there is a critical lack of data that is required to compute opacities and the subsequent theoretical atmosphere for high-metallicity atmospheres. This is because high-metallicity atmospheres are expected to contain larger fractional quantities of H₂O, CO, CO₂, and CH₄, relative to H₂-dominated systems that have been the focus of the majority of previous observing campaigns. Therefore, they require fundamentally different pressure-broadening parameters that are currently lacking. Nevertheless, ignoring the impact of these parameters will lead to errors in the calculation of the planet’s energy budget, as well as errors in the ultimate atmospheric spectra. We will present an overview of our team’s efforts to fill this gap by computing the theoretical broadening coefficients relevant to the super-Earth to sub-Neptune temperature range. The importance of these results will be discussed and their impact on exoplanet radiative transfer modeling of objects from space-based telescopes will be discussed.