Polarization spectroscopy (PS) is a nonlinear pump-probe technique that utilizes a change in the polarization of the probe beam, due to anisotropic Zeeman state populations created in the target molecule by the pump beam. The line strength and line shape of a PS signal is dependent on the structure and dynamics of the molecule. Here we present a two-color polarization spectroscopy (TCPS) technique, with independent spectral tuning of the pump and probe beams, for investigating collision induced transitions (CITs) in the A²Σ⁺-X²Π (0-0) band of nitric oxide (NO) at 295 K and 1 atm. We distinguish CITs from shared level transitions (SLTs), which occur when the transitions excited by pump and probe beams share a common upper and/or lower level. In TCPS, CITs only occur when Zeeman state anisotropy (ZSA) is preserved to some extent during state-to-state rotational transfer. In these NO TCPS studies, a circularly-polarized pump beam is tuned to create a ZSA in the NO molecules at either low or high J level. A weak probe beam is then scanned over a range of transitions close to the pump frequency in order to probe SLTs and CITs. Gas mixtures of 1% NO in the buffer gases He, N₂, and Ar were selected for the experiments to investigate the dependence of CIT on collision species. The strongest CITs were observed in the He buffer gas and the weakest CITs were observed in the Ar buffer gas. A time-dependent density matrix model incorporating a modified-exponential-gap-law-based collisional submodel was developed to investigate the state-to-state ZSA retention rate of NO with different colliding partners. A key result is that the spin-flip transition rate in the A²Σ⁺ level is significant at low J, while being almost zero at high J (J > 10). These results will provide useful information for comparison with ab initio potential energy surface collision studies. l0pt Figure

r0pt Figure We present absorbance measurements of the full fundamental ro-vibrational band of nitric oxide in nitrogen bath (NO/N₂) from 1700 to 2000 cm⁻¹ at pressure and temperature conditions of 20-34 atm and 293-802 K, respectively. These measurements were taken with a narrow-linewidth, broad-scan EC-QCL laser aligned through a high-pressure, high-temperature static cell. The results were compared to a line mixing model utilizing relaxation matrix formalism and the modified exponential gap (MEG) fitting law that was previously developed using absorbance measurements limited to the R-branchAlmodovar, C.A., "Infrared Laser Absorption Spectroscopy of Nitric Oxide for Sensing in High-Enthalpy Air," Ph.D. diss., (Stanford University, 2019) Improved knowledge from full-band absorbance measurements, including the P and Q-branches, indicates the need to account for inter-branch line mixing effects which were omitted in the previous model. A modified MEG line mixing model is presented with additional inter-branch coupling transfer rates and their associated scaling factors, showing stronger agreement with the measured spectra.

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Footnotes:

Almodovar, C.A., "Infrared Laser Absorption Spectroscopy of Nitric Oxide for Sensing in High-Enthalpy Air," Ph.D. diss., (Stanford University, 2019).

Line-by-line spectroscopic databases containing CO₂, such as HITRAN, include so many transitions and bands that it is intractable to provide reference data for every transition and relevant collisional partner. To address this dilemma, theoretical line parameters enabling widespread coverage need to be validated in terms of accurate spectroscopic measurements having well-established uncertainties. In this work, we present frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS) measurements of CO₂ (30012) ← (00001) band transitions up to J^′′ = 50. These data correspond to foreign broadening by four gas mixtures: air, N₂, and two Ar-enriched synthetic air samples, with each sample containing CO₂ at a mole fraction near atmospheric levels. The measured O₂, N₂, and Ar foreign broadening parameters in addition to advanced line shape parameters were simultaneously determined using a multi-spectrum fitting algorithm which used constraints for pressure, temperature, and sample composition. These results were compared to theoretical and experimental broadener-specific line shape parameters from the literature. Additionally, requantized classical molecular dynamic simulation results were explored as constraints to enable differentiation between narrowing mechanisms in finite signal-to-noise ratio spectra.

Characterizing and modelling the atmospheric CO₂ with a sub-percent accuracy necessitates high-quality spectroscopic lineshape parameters. For this goal, we collected the best experimentally and theoretically measured lineshape coefficients of CO₂ lines broadened by air and CO₂ to create sets of semi-empirical models for updating all the transitions of the HITRAN database [1]. Based on the available data, we estimated the air- and CO₂- broadening coefficients, their associated temperature exponents, and the speed dependence of the broadening together with its temperature dependence for every transition in HITRAN. Furthermore, the semi-empirical approach proposed by Hartmann 2009 [2] (trained by most reliable experimental data) is used to calculate the line shifts of air- and self- broadened CO₂ absorption lines. The updated data will be provided on www.hitran.org and will be used in the next edition of HITRAN. It is notable that thanks to the relational structure of the HITRAN database we provide separate consistent and complete sets of parameters in Voigt and speed-dependent Voigt parametrizations. Finally, the updated lineshape parameters are used for calculating the first-order line mixing using the program developed by Lamouroux et al. [3] which will also be provided in HITRAN 2020. For verifying the semi-empirically calculated lineshape parameters, the laboratory spectra measured by Dr. Keeyoon Sung at the Jet Propulsion Laboratory (JPL) is used to compare with the calculation of the absorption coefficient by HITRAN Application Programming Interface (HAPI) [4] which is now equipped with necessary functionality. [1] JQSRT, 203 (2017) 3-69. [2]JQSRT, 110 (18) (2009) 2019-2026. [3] JQSRT, 151 (2015) 88-96. [4] JQSRT, 177 (2016) 15-30.

Accurate absorption models for gases at high pressure and temperature are needed for applications in both planetary and combustion science. Absorption models for these conditions require complete databases of temperature-dependent line shape parameters as well as temperature-dependent models for higher-order collisional effects that manifest at high pressure (e.g. line mixing, collision-induced absorption, finite duration collisions). In support of this goal, we present new measurements of CO₂ at pressures up to 25 bar and temperatures up to 981 K recorded with a broadband, high-resolution dual frequency comb absorption spectrometer in a specialized gas cell. We analyze the measured spectra using a multispectrum fitting approach to extract temperature-dependent line shape parameters for bands in the 6800 – 7000 cm⁻¹ range using both a Voigt and speed-dependent Voigt profile. We compare the new parameters to prior measurements and calculations, particularly those adopted for the HITRAN2020 database. Finally, using the new line shape parameters, we expand our absorption model to include line mixing effects, and discuss the performance of the line mixing model at high pressure and temperature. These results improve absorption models for CO₂ in high-pressure and -temperature conditions and, more broadly, inform strategies for modelling high-pressure, high-temperature spectra in combustion and planetary science.

Fourier transform spectra include the non negligible contribution of the instrumental line shape (ILS) of the spectrometer, even at high resolution. To avoid biasing the information retrieved from these spectra, an accurate modeling of the actual ILS of the spectrometer is required. Several methods have been developed to retrieve the ILS from Fourier transform spectra of isolated lines at low pressure, either partiallyD. Jacquemart, M. Chaboud, J. Quant. Spectrosc. Radiat. Transf. 119 (2013) 95-103.r completely,F. Hase, T. Blumenstock, C. Paton-Walsh, Appl. Opt. 38 (1999) 3417-3422.^,C. Bernardo, D.W.T. Griffith, J. Quant. Spectrosc. Radiat. Transf. 95 (2005) 141-150.ncluding its wavenumber dependence for the ACE−FTS.C.D. Boone, P.F. Bernath, J. Quant. Spectrosc. Radiat. Transf. 230 (2019) 1-12.he present method builds upon the work of Boone and Bernath.^5 It replaces the modeling of the wavenumber dependence of the ILS adapted to the ACE−FTS instrument, which turned out not to be appropriate for high resolution laboratory Fourier transform spectrometers, and accounts for the loss of information on the ILS resulting from the molecular line shape. The method, implemented in a simple python script, will be presented and its performance compared with linefit.^3 C. Bernardo, D.W.T. Griffith, J. Quant. Spectrosc. Radiat. Transf. 95 (2005) 141150.iC.D. Boone, P.F. Bernath, J. Quant. Spectrosc. Radiat. Transf. 230 (2019) 112.T

r0pt Figure There is considerable ambiguity in the collisional broadening mechanism for the resonance doublets of alkali metals, e.g., potassium near 0.77 μm. The presence of alkali metals within stellar atmospheres of brown dwarfs calls for accurate modeling on the effect of collisional broadening to better interpolate the spectra measured by space telescopes. We present an easy approach to generate potassium vapor in the laboratory at conditions similar to the effective surface temperatures ( ∼ 1000 K) of brown dwarfs. High-temperature potassium/argon mixtures are produced in a shock tube via shock-heating seeded potassium chloride salts. This seeding approach is effective over 1100 - 1900 K and is very safe, compared with the handling of highly reactive potassium samples. The absorption line shapes of both K I resonance doublets (D1: 770 nm; D2: 767 nm) are studied with argon as the collisional partner. The measured spectra are well-modeled with Voigt profiles, and the measured broadening parameters, i.e., argon broadening coefficients and pressure shift coefficients, are well-fit with power-law relationships in the range of test temperatures.

l0pt Figure In 2016, Iwakuni et al. measured the ν₁ + ν₃ vibrational combination band of acetylene using a dual-frequency comb based spectrometer. They reported an alternation in the self-pressure broadening coefficients of even and odd rotational levels, which correspond to the para- and ortho- nuclear spin states. This can occur if relaxation involving resonant energy transfer between molecules with common nuclear spin symmetry is important, because ortho-ortho collisions are statistically more probable than para-para ones. Subsequently several authors have disputed these findings. Our recently published results, using a frequency-comb stabilized laser spectrometer found no experimental evidence of the reported effect in the R(8)−R(13) lines of the band. Analysis of the data needed careful accounting of weak underlying absorptions due to hot-bands and lower abundance isotopomer lines. Detailed modeling of Iwakuni et al. results suggest the alternation is an artifact of the fits neglecting collisional narrowing in a strongly absorbing regime. Further low temperature measurements have been planned to try to limit influence of the hot-band lines.

Methane is present in the atmosphere of a large number of objects in the Solar system and beyond. Probing CH₄ in these environments using optical remote sensing relies on the availability of appropriate information characterizing its spectrum. The present work aims to provide the spectroscopic information required to describe the effects of pressure on the infrared spectrum of methane in CO₂ dominated atmospheres, to compensate for the scarcity of the information currently available.M. Gharavi, S.G. Buckley, J. Mol. Spectrosc. 229 (2005) 78-88.^,L. Fissiaux et al., J. Mol. Spectrosc. 297 (2014) 35-40.^,O.M. Lyulin et al., J. Quant. Spectrosc. Radiat. Transf. 147 (2014) 164-170.^,E. Es−Sebbar, A. Farooq, J. Quant. Spectrosc. Radiat. Transf. 149 (2014) 241-252.^,J. Manne et al., J. Quant. Spectrosc. Radiat. Transf. 191 (2017) 59-66.^,B. Vispoel et al., J. Quant. Spectrosc. Radiat. Transf. 360 (2019) 1-6.igh resolution Fourier transform spectra of methane diluted in carbon dioxide at total pressures up to 800 hPa have been recorded at room temperature in the pentad region around 3.3 L. Fissiaux et al., J. Mol. Spectrosc. 297 (2014) 3540.E. Es-Sebbar, A. Farooq, J. Quant. Spectrosc. Radiat. Transf. 149 (2014) 241252.B. Vispoel et al., J. Quant. Spectrosc. Radiat. Transf. 360 (2019) 16.HD.C. Benner et al., J. Quant. Spectrosc. Radiat. Transf. 53 (1995) 705721.a

New measurements of the a¹∆_g ← X³Σ_g⁻ band of oxygen (¹⁶O₂) at 1.27 μm will be presented, improving on a previous study [Mendonca et al., Atmos. Meas. Tech. 12, 35-50 (2019)]. Spectra were acquired by frequency-stabilized cavity ring-down spectroscopy over a 160 cm⁻¹ wave number range (7792 cm⁻¹ to 7952 cm⁻¹). The frequency axis was anchored to a Cs-clock-referenced optical frequency comb through a heterodyne beat note between the comb and the probe laser at about 20 points across the wave number range. The probe laser was phase locked to the frequency comb prior to measuring the beat note frequency in order to improve the accuracy, yielding a 10-Hz uncertainty in the ring-down cavity free spectral range and a 50-kHz absolute frequency uncertainty for all mode orders. Six air-broadened spectra were recorded, at pressures ranging from 3.3 kPa to 100 kPa. They were analyzed with custom multi-spectrum fitting software based on the HAPI python library, using the speed-dependent Nelkin-Ghatak profile. The resulting line shape parameters reveal important discrepancies with the HITRAN2016 values. These results will also be compared to those reported in recent studies by the Grenoble group [Konefał et al., JQSRT 241, 106653 (2020) and Tran et al., JQSRT 240, 106673 (2020)].

The study and modelling of the N₂-perturbed lines in O₂ is of great importance for Earth's atmosphere studies. However, collisional systems with the ground-state molecular oxygen, O₂ (X³Σ⁻_g), either as perturbing or active molecule, are difficult to handle because of its non-zero spin that needs to be taken into account in the description of quantum scattering. Here, we present the methodology as well as the results of our quantum scattering calculations and the line-shape parameters for the O₂ immersed in molecular nitrogen bath. It is the first theoretical ab initio investigation of this collisional system in the context of the shapes of molecular lines. The PES for this study was constructed automatically using the AUTOSURF code Quintas-Sánchez, Ernesto, and Richard Dawes. ÄUTOSURF: A freely available program to construct potential energy surfaces." Journal of chemical information and modeling 59, no. 1 (2018): 262-271 The data provided through this investigation is important for the terrestrial atmospheric measurements and can be used for populating the spectroscopic databases such as HITRAN or GEISA.

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Footnotes:

Quintas-Sánchez, Ernesto, and Richard Dawes. ÄUTOSURF: A freely available program to construct potential energy surfaces." Journal of chemical information and modeling 59, no. 1 (2018): 262-271.

r0pt Figure We present fully ab initio calculations of second-overtone rovibrational line shapes of carbon monoxide perturbed by argonG. Kowzan, H. Cybulski, P. Wcisło, M. Słowiński, A. Viel, P. Masłowski, F. Thibault, PRA 102, 012821 (2020). The quantum mechanical scattering problem between the CO molecule and the Ar atom is solved numerically for an ab initio interaction potential. We use the generalized Hess methodL. Monchick and L. W. Hunter, JCP 85, 713 (1986).o determine the spectroscopic cross sections which describe the effect of collisions on spectral lines. These cross section are then used to determine line-shape parameters of the Hartmann-Tran profile, its β-correctedM. Konefał, M. Słowiński, M. Zaborowski, R. Ciuryło, D. Lisak, P. Wcisło, JQSRT 242, 106784 (2020).ersion and the speed-dependent billiard ball profile (SDBBP). We compare the generated line shapes with high-quality experimental line profilesG. Kowzan, K. Stec, M. Zaborowski, S. Wójtewicz, A. Cygan, D. Lisak, P. Masłowski, R. S. Trawiński, JQSRT 191, 46 (2017).btained at five pressures between 0.01 and 1 atm and obtain subpercent agreement in the whole pressure range. We show that the remaining discrepancies are mostly due to residual errors of ab initio pressure broadening and shift. We use the P(9) line data to compare two potential energy surfaces. We explain the differences in obtained pressure broadening and shift coefficients based on the properties of the surfaces.

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Footnotes:

G. Kowzan, H. Cybulski, P. Wcisło, M. Słowiński, A. Viel, P. Masłowski, F. Thibault, PRA 102, 012821 (2020).. L. Monchick and L. W. Hunter, JCP 85, 713 (1986).t M. Konefał, M. Słowiński, M. Zaborowski, R. Ciuryło, D. Lisak, P. Wcisło, JQSRT 242, 106784 (2020).v G. Kowzan, K. Stec, M. Zaborowski, S. Wójtewicz, A. Cygan, D. Lisak, P. Masłowski, R. S. Trawiński, JQSRT 191, 46 (2017).o

We show in this talk that requantized classical molecular dynamics simulations (rCMDSs) are capable of predicting various refined spectral-shape parameters of absorption lines of linear molecules such as CO2, N2O and O2 with high precision. Combining CMDSs and a requantization procedure, we computed the auto-correlation function of the dipole moment responsible for the absorption transition. Its Fourier-Laplace transform directly yields the spectrum. Calculations were made for various temperature conditions, at 1 atm and for a large range of Doppler widths, from the near-Doppler to the collision-dominant regimes. For each temperature and each line, the spectra calculated for various Doppler widths were simultaneously fit with a refined line-shape model. The latter takes into account the effects of the speed dependent collisional line broadening, the Dicke narrowing, and the collisional line mixing. The obtained parameters were then directly compared with those deduced from high-precision measurements. The results show very good agreements, even for those parameters whose influence on the spectra is very small. Good agreement is also obtained between measured and predicted temperature dependences of these parameters. This demonstrates that rCMDS is an excellent tool, highly competitive with respect to high quality measurements for precise line-shape studies.