Several years ago, Iwakuni et al. reported an unexpectedly strong ortho-para-dependence to the self-pressure broadening in the ν₁ + ν₃ vibrational band of acetylene. Such an effect can arise because ortho-ortho collisions are statistically more probable than para-para ones and resonant energy transfer processes can make like-molecule collisions more efficient in state-changing, lifetime-shortening, collisions. Subsequently several papers have disputed the observation on the basis that the experimental sensitivity could not be sufficient, and the approximate Voigt lineshape model used in the analysis would lead to systematic errors. However, there has been no reported independent experimental work to verify the results or investigate the refutations. Our group previously reported a very accurate and precise measurement of the self-pressure broadening for the P(11) transition in the same band, using a comb-stabilized laser spectrometer. We have now resurrected this instrument, and measured additional R-branch lines to investigate the controversial results described above. Measurements of the R(12)−R(15) transitions, which showed significant ortho-para differences in the original work, have been made at pressures ranging from 200 mTorr to 150 Torr. We fit the data to both Voigt and speed dependent Voigt (SDV) lineshape model to determine the pressure broadening coefficients and investigate their rotational and nuclear-spin dependence. Acknowledgment: This material is based upon work supported by the U.S. Department of Energy, Office of Science, Division of Chemical Sciences, Geosciences and Biosciences within the Office of Basic Energy Sciences, under Award Number DE-SC0018950.

Vacuum ultraviolet spectroscopy was used to explore the density dependence of supercritical carbon dioxide electronic absorption spectra over the wavelength range 145.5-200 nm at 34.5 ^°C. Pressure was varied from 19 to 137 bar, giving a corresponding density range of 0.036-0.767 g cm⁻³. The vibronic structure inherent to the spectrum is apparent at the lowest densities, but gradually diminishes in magnitude with increasing density. At a density of 0.595 g cm⁻³ the structure is no longer apparent. This loss of detail cannot be explained by collisional broadening or dimerization, and we suggest gradual perturbation of the monomer electronic and vibrational structure with increasing density, similar to that observed in recent studies of supercritical water.

Accurate spectroscopic models are required for remote sensing missions that use spectroscopic methods to interrogate atmospheric composition. The oxygen A-band (762 nm) is utilized for determination of air mass, solar pathlength and surface pressure in remote sensing applications due to the uniform concentration of molecular oxygen throughout the atmosphere and the spectral isolation of the band. NASA’s OCO-2 satellite seeks to retrieve atmospheric carbon dioxide concentrations with an accuracy of 0.25%, placing stringent demands on our knowledge of the A-band spectral parameters. Current limitations in the A-band spectroscopic models, primarily from the treatment of line mixing (LM) and collision induced absorption (CIA), remain a significant source of error in carbon dioxide column retrievals. LM is manifested as an intensity exchange due to collisional population transfer between closely spaced energy levels while CIA appears as a broad, weak continuum absorption feature arising from transient dipoles induced by molecular collisions. Photoacoustic spectroscopy, a zero-background technique with a large dynamic range, is an ideal method to observe these effects which become increasingly prominent at elevated pressures. We have developed a high precision (SNR 10,000), broadband photoacoustic spectrometer for recording full A-band spectra at room temperature over a wide range of pressures (300-3000 Torr). Intensity exchange due to LM is observed in these unsaturated, high SNR spectra, and the weak baseline CIA profile can be extracted without interferences from instrumental background effects. Results from multispectrum fits of this data with non-Voigt line shapes showing insufficiencies in current A-band models will be presented.

The collision-induced absorptions (CIA) of O₂ and dry air were measured in the near-infrared and visible region, covering the O₂ B-band at 687 nm and double transitions in the 630 and 577 nm region. A newly customized 1 m pathlength high-pressure cell was developed and configured to a Fourier transform spectrometer, Bruker 125HR, at the Jet Propulsion Laboratory. A super luminous cutting-edge Laser-Driven Light Source (LDLS), was also used to improve the photon flux offered by the spectrometer. A series of spectra of pure O₂ and dry air were obtained at various pressures up to 131 bars at room temperature. For the CIA of O2 B-band region, the monomer resonance absorption contribution to the observed spectra has been subtracted by simulating their absorption with a speed-dependent Voigt line shape profile with line mixing effects taken into account. The remaining absorption component was interpreted as the CIA component in the region. The integrated absorption coefficient was measured to be 0.024(6)×10⁻⁴ cm⁻¹/Amag² for the O₂ B-band region, which are significantly lower than early measurements. For the two double transition bands in the 630 and 577 nm regions, however, the integrated CIA from this work were measured to be 2.50(14) and 3.17(18)×10⁻⁴ cm⁻²/Amag², respectively, which are significantly higher than their corresponding early measurements. For dry air, the integrated CIA were measured to be 0.10(2) and 0.15(2)×10⁻⁴ cm⁻²/Amag², respectively, for the 630 and 577 nm region, with no appreciable contribution from the O₂-N₂ pairs in this work. The results are compiled in electronic supplements.^{& &} Government sponsorship acknowledged.

Although the Voigt profile (VP) has long been used as a standard spectroscopic line profile, more stringent demands in high-resolution spectroscopy now require the application of other profiles that account for physics not captured by the VP. The Hartmann-Tran Profile (HTP) [1] was recommended by an IUPAC task group to be a standard for high-resolution spectroscopy because it parameterizes higher-order physical effects, is computationally efficient, and reduces to the VP and other widely used profiles as limiting cases [2]. As advanced line profiles such as the HTP are adopted by more researchers to model or predict absorption spectra, it is important to understand the limitations that data quality has on the ability to retrieve physically meaningful parameters from least-squares fits of assumed profiles to measurements with finite signal-to-noise ratio (SNR). In this work, synthetic, noisy spectra were simulated using the HITRAN Application Programming Interface (HAPI) [3] across a range of parameters, SNR, and spectral sampling interval. The HTP was fit to these spectra to determine how simulated conditions affect fitted parameter uncertainty and correlations between parameters. We also investigated under what circumstances and constraints the parameters derived from fitting the HTP adequately model the full HTP. 1. Tran, H., N. Ngo, and J.-M. Hartmann, Efficient computation of some speed-dependent isolated line profiles. Journal of Quantitative Spectroscopy and Radiative Transfer, 2013. 129: p. 199-203. 2. Tennyson, J., et al., Recommended isolated-line profile for representing high-resolution spectroscopic transitions (IUPAC Technical Report). Pure and Applied Chemistry, 2014. 86(12): p. 1931-1943. 3. Kochanov, R.V., et al., HITRAN Application Programming Interface (HAPI): A comprehensive approach to working with spectroscopic data. Journal of Quantitative Spectroscopy and Radiative Transfer, 2016. 177: p. 15-30.

Absorption spectra of hot methane were recorded in the 2300-3200 cm⁻¹ spectral region (ν₃ mode) using a Bruker 120/125 HR Fourier transform spectrometer. Methane was heated in a quartz cell in a tube furnace at 295, 473, 673, 873 and 1073 K. Line broadening of the methane spectra was investigated by adding 50, 150 and 400 Torr of H₂ as a broadening gas to 0.5 Torr of methane. A preliminary spectral fit of the methane data was performed using Voigt line shape functions with the WSpectra program. The temperature and pressure dependence of the line broadening parameters were studied; additional spectra are needed for more temperatures and pressures. A more sophisticated analysis using non-Voigt line shapes and a multi-spectral fit will be carried out.

Infrared line positions and pressure-broadening data of atmospheric key absorbers H₂O and strongly impact the interpretation of exoplanetary observational data. The inaccuracy of absorption cross-sections (opacity data) biases atmospheric radiative transfer modeling. The detection of CH₄, for instance, is still under debate, despite extensive endeavors to model the chemical composition of exoplanetary atmospheres. To this end, we are carrying out the following projects. First, high-resolution high-temperature H₂-pressure-broadened spectra are recorded for the CH₄ ν₃-band P-branch. Measured linewidths for 112 transitions between 2840 and 3000 cm⁻¹ with temperature and pressures ranging between 300 and 700 K, and 10 and 933 Torr, respectively, were used to find rotation- and tetrahedral-symmetry-dependent coefficients for pressure and temperature broadening and pressure-induced lineshifts. Second, we are currently investigating the impact on H₂O absorption cross-sections of various line lists (e.g., POKAZATEL vs. BT2). We assess the potential bias when interpreting the cross-correlation function through Earth-based observations. We report our progress in measuring high-temperature H₂O line positions.

We present results of an extensive analysis of the CO 1←0 band in 40 spectra of pure carbon monoxide and carbon monoxide mixed with air recorded at temperatures ranging between 79 K and room temperature. All spectra were recorded using the 1-m McMath-Pierce Fourier Transform spectrometer located at Kitt Peak, AZ, USA and two temperature-controlled gas cells. The analysis was carried out using multispectrum fitting softwareD. C. Benner, C. P. Rinsland, V. Malathy Devi, M. A. H. Smith and D. A. Atkins, JQSRT 53 (1995) 705-721.nd the Voigt, speed-dependent Voigt and Rautian line shape models. When using the Rautian model, we employed calculated narrowing parameters obtained from computed diffusion constantsJ. O. Hirschfelder, C. F. Curtiss and R. B. Bird, Molecular theory of gases and liquids, New York, Wiley and Sons, 1952.or each of the absorber-perturber pairs CO-CO, CO-N₂ and CO-O₂. The experimentally retrieved temperature dependences of the line shape parameters are been compared with previous published results and with the results of calculations for CO-N₂. We thank D. Chris Benner for the Labfit software. The work of V. M. Devi was funded by NASA grants and contracts, and the research by M. A. H. Smith was performed as part of her former employment at NASA Langley Research Center. No official endorsements are intended or implied. N. Islam and A. Predoi-Cross have been funded by NSERC. S. Ivanov received financial support from the Ministry of Science and Higher Education within the State assignment FSRC "Crystallography and Photonics" RAS and Russian Science Foundation (Project No.18-55-16006).

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Footnotes:

D. C. Benner, C. P. Rinsland, V. Malathy Devi, M. A. H. Smith and D. A. Atkins, JQSRT 53 (1995) 705-721.a J. O. Hirschfelder, C. F. Curtiss and R. B. Bird, Molecular theory of gases and liquids, New York, Wiley and Sons, 1952.f