The role of velocity changing collisions (VCCs) in pressure-dependent line shapes is revisited, highlighting their contributions to pressure broadening in sub-Doppler saturation line shapes and the conditions required for collisional narrowing in isolated Doppler- and pressure-broadened lines. As reported at last year's meeting (Paper WJ06, 72^nd ISMS), we have observed the self-broadening of sub-Doppler saturation dip absorption lines in the v₁+v₃ band of acetylene near 1.5μm in frequency comb-referenced measurements. The saturation line shapes are well described by Voigt functions with a fixed, narrow Gaussian component and a Lorentzian component that increases linearly with pressure up to 0.04 mbar. This sub-Doppler pressure broadening exceeds the normal pressure broadening of a full Doppler line observed at higher pressures. Velocity changes following large cross-section, elastic, collisions are dominated by a sharply spiked exponential cusp in the laboratory-frame collision kernel. The VCCs will contribute to the total broadening when the typical change in Doppler detuning associated with small angle elastic collisions exceeds the pressure-dependent homogeneous line width associated with inelastic damping. At higher pressures, the homogeneous width becomes larger than this collisional frequency shift, and the additional damping effect of VCCs becomes negligible. The pressure at which the change in slope of the line width vs. pressure will occur depends on details of the elastic collision kernel. A Monte Carlo sampling model of elastic and inelastic collision rates and cusp-like elastic collision kernels has been developed to generate electric field time correlation functions whose real Fourier transforms depict the pressure dependent line shapes. Useful physical insights follow. In order to produce collisional (Dicke) narrowing, multiple velocity changing collisions must generate large changes in the Doppler shift of a given absorbing molecule prior to its first inelastic collision.

Methane has been observed in brown dwarfsB. Oppenheimer, S. Kulkarni, K. Matthews et al., Astrophys. J. (1998), 502(2), 932-943.nd planetary atmospheres, including planets in our solar systemT. Owen, R. Cess, Astrophys. J. (1975), 197, L37-L40.nd extrasolar planets.M. Swain, G. Vasisht, G Tinetti et al., Nature (2008), 452, 329-331.ethane is also a potent greenhouse gasM. Khalil, Annu. Rev. Energy Environ. (1999), 24, 645-661.nd relevant to ozone formation and depletion in Earth’s atmosphere.O. Boucher, P. Friedlingstein, B. Collins et al., Environ. Res. Lett. (2009), 4(4), 044007.s the simplest stable hydrocarbon, methane is also a benchmark for state-of-the-art ab initio calculations.A. Nikitin, M. Rey, V. Tyuterev, J. Quant. Spectrosc. Radiat. Transf. (2017), 200, 90-99.hile methane is a strong absorber due to its characteristically large transition dipole moments, transition frequencies were historically limited by Doppler broadening, and many frequencies are still known only to Doppler-limited precision. We have constructed a double-pass saturation experiment to perform sub-Doppler spectroscopy of rovibrational transitions of methane. With the accuracy provided by optical frequency combs, we have measured 22 methane transitions from the ν₃ band in the 3 μm region to MHz-level uncertainty, improving the accuracy of the rest frequencies by at least an order of magnitude. This data can be used for higher-precision models of methane as an ab initio benchmark.

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Footnotes:

B. Oppenheimer, S. Kulkarni, K. Matthews et al., Astrophys. J. (1998), 502(2), 932-943.a T. Owen, R. Cess, Astrophys. J. (1975), 197, L37-L40.a M. Swain, G. Vasisht, G Tinetti et al., Nature (2008), 452, 329-331.M M. Khalil, Annu. Rev. Energy Environ. (1999), 24, 645-661.a O. Boucher, P. Friedlingstein, B. Collins et al., Environ. Res. Lett. (2009), 4(4), 044007.A A. Nikitin, M. Rey, V. Tyuterev, J. Quant. Spectrosc. Radiat. Transf. (2017), 200, 90-99.W

Methane (CH₄) is the dominant carbon-bearing molecule in terrestrial and exoplanetary atmospheres where the temperature is below 1000 K. Therefore, knowing its pressure-induced H₂-broadened absorption cross section is fundamental for exoplanetary atmospheric modeling. In this study, the pressure-induced H₂-broadening coefficients of CH₄ are determined in the spectral regions  2800-3200 (ν₃) and 1200-1400 cm⁻¹ (ν₄). The laboratory transmission spectra in this study were recorded at high resolution (i.e., 0.005 and 0.01 cm⁻¹) at room temperature with an FTIR 125HR Bruker spectrometer at Lawrence Berkeley National Laboratory. The CH₄ pressure was constant during the entire experiment (29 mtorr), and elevated H₂ pressures were used in the range 100-700 torr. The Lorentzian coefficients are determined by a nonlinear regression approach in order to model the relationship between the linewidth and its corresponding pressure. Our preliminary results show that the Lorentzian coefficients of different lines in these two bands fall in the range 0.06-0.09 cm⁻¹/atm, consistent with the previous available measurements. Atmospheric modeling will be employed using exoplanet forward transmission modeling to highlight the importance of H₂-broadening of CH₄ to exoplanetary observations.

Collision induced absorption (CIA) coefficients of the a¹∆_g-X³Σ⁻_g(v=0-0) band of oxygen have been measured using cavity ring down spectroscopy (CRDS) technique at room temperature. More precisely, the B_O2−O2, B_O2−N2 and B_O2−Air coefficients have been determined with a reduced uncertainty from series of low density spectra (from 0.36 to 0.85 amagat) of pure oxygen and N₂+O₂ mixture with O₂=20.95%. For that 12 distributed feed-back laser diodes were used below 7920 cm⁻¹together with an external cavity diode laser above this wavenumber. We particularly paid attention to the base line stability (2 ×10⁻¹⁰ cm⁻¹) during the entire measurements. CIA was obtained from the difference between the absorbing samples spectra and argon spectra recorded for the same densities after removal of the local contribution of the absorption lines. The low densities at which the spectra were recorded were very useful to reliably remove this local contribution. The retrieved coefficients were compared to the CIA reported in HITRAN2016. A good overall agreement is found but differences between 5 and 8% for B_O2−Air coefficients are observed below 7850 cm⁻¹.

Because of the constant mixing ratio of molecular oxygen (O₂) within the Earth’s atmosphere, the O₂ A-band is commonly used in satellite and remote sensing measurements as a measure of the airmass. A recent collaborative effort has produced a self-consistent integrated spectroscopic model for the O₂ A-Band that simultaneously accounts for high-order line-shapes, line mixing (LM), and collisional induced absorption (CIA).B. J. Drouin, D. C. Benner, L. R. Brown, et al., Multispectrum analysis of the oxygen A-band, J. Quant. Spectrosc. Radiat. Transfer, 2017, 186: p. 118-138.his model has improved OCO-2 mission retrievals of dry air CO₂, however, limitations in existing spectroscopic models still lead to airmass dependent biases. Currently, model development is limited by a lack of high resolution experimental data at low temperatures and in the R-branch. To address this, measurements of the entire O₂ A-band were recently made with a variable-temperature cavity ring-down spectrometer (CRDS) over a range of temperatures, pressures, and molar fractions. Because of the limited dynamic range of the CRDS system, at high molar fractions of O₂ saturation can occur at the line cores of strong transitions. Therefore, a range of molar fraction O₂ samples were employed. Low mole fraction data, which was unaffected by saturation provided information on the temperature dependence of high-order line-shape parameters. Conversely, high molar fraction data provided information on LM and CIA effects that dominate absorption in the troughs between saturated transitions. By combining this high-resolution experimental data, that covers both the entire O₂ A-Band as well as a range of temperatures, with existing datasets, these results aim to improve on LM and CIA models for the next iteration of the global O₂ A-Band model.

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Footnotes:

B. J. Drouin, D. C. Benner, L. R. Brown, et al., Multispectrum analysis of the oxygen A-band, J. Quant. Spectrosc. Radiat. Transfer, 2017, 186: p. 118-138.T

Accurate parameters of the N₂O transitions in the 1.5μm region are needed for monitoring global N₂O concentration in the atmosphere. The strongest band in this region is the 0003-0000 band. In HITRAN database, some parameters of this band are given by calculation, others are given by experiments but they are obtained by the Voigt profile, which is now well known can lead to significant deviations. The ro-vibrational transitions of the 0003-0000 band with line intensities in the order of 10⁻²⁴ to 10⁻²³cm⁻¹/(molecule·cm⁻²) have been recorded using a laser-locked cavity ring-down spectrometer with high sensitivity as well as high precision. The positions were determined with an uncertainty of sub-MHz. The line intensities and Nitrogen induced pressure broadening coefficients were also derived with accuracies better than 0.8% and 1%, respectively. Comparisons of the line parameters determined in this work with literature experimental values and those from HITRAN2016 database are given.

The HITRAN Application Programming Interface (HAPI)Kochanov RV, Gordon IE, Rothman LS et al. JQSRT 2016;177:15–30. doi:10.1016/j.jqsrt.2016.03.005.s a powerful tool for working with spectroscopic data in the gas phase. HAPI provides access to the capabilities of the HITRANonline (http://hitran.org) web information system with the recent edition of the HITRAN2016 spectroscopic databaseGordon IE, Rothman LS, Hill C, Kochanov RV, Tan Y et al. JQSRT 2017;203:3-69. doi:10.1016/j.jqsrt.2017.06.038. Besides an access to HITRANonline, HAPI allows working with user-supplied data. Among the capabilities are data filtering and analysis, as well as modeling of gas absorption with the fine tuning of many parameters (gas mixture, path length, instrumental function, temperature, and pressure). In this talk we present the update for HAPI (v.2.0) which has the following features: 1) access to line-by-line spectroscopic transitions and experimental cross-sections from HITRAN2016; 2) access to the metadata for molecules from the line-by-line part, and more than 300 molecules from the cross-section part, as well as for the database bibliography; 3) seamless use of the foreign broadening and shifting parameters, and non-Voigt line profiles, relevant for atmospheric and planetary applications; 4) use of the custom CPF implementations; 5) updated partition sums from the recent TIPS softwareGamache RR, Roller C, Lopes E, Gordon IE, Rothman LS et al. JQSRT 2017;203:70–87. doi:10.1016/j.jqsrt.2017.03.045.overing wider temperature ranges; 6) line mixing support. The new version features HAPIEST (HAPI and Efficient Spectroscopic Tools) – a portable graphical user interface providing access to HAPI features. HAPI v.2.0 is available at the official HITRANonline site as well as through the Github repository (https://github.com/hitranonline/hapi). The HAPIEST open source package with binary installers will be available at HITRANonline upon release. This effort is supported through the NASA AURA (NNX 17AI78G) and NASA PDART grants (NNX16AG51G).

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Footnotes:

Kochanov RV, Gordon IE, Rothman LS et al. JQSRT 2016;177:15–30. doi:10.1016/j.jqsrt.2016.03.005.i Gordon IE, Rothman LS, Hill C, Kochanov RV, Tan Y et al. JQSRT 2017;203:3-69. doi:10.1016/j.jqsrt.2017.06.038.. Gamache RR, Roller C, Lopes E, Gordon IE, Rothman LS et al. JQSRT 2017;203:70–87. doi:10.1016/j.jqsrt.2017.03.045.c

The absorption cross section of water vapor in the blue-violet range (415-460 nm) is currently not well known, and many weak spectral lines are not included in either the HIgh resolution TRANsmission molecular absorption (HITRAN) database or its HIgh TEMPerature companion, HITEMP. Direct measurements of the absorption cross section of water vapor in this region have been limited by the slant column density (SCD) of gaseous water molecules achievable in a laboratory setting. We use cavity-enhanced differential optical absorption spectroscopy (CE-DOAS) to generate water vapor SCDs comparable to those in field measurements. Our cavity consists of high-reflectivity (R > 0.99995) mirrors separated by  80 cm to realize effective path lengths up to 16 km; water vapor is generated from deionized water in a double-bubbler system. Broadband light sources (LEDs) with peak intensities at 420 and 455 nm allow us to measure multiple lines at moderately high spectral resolution (0.15 nm). The first spectra were measured at room temperature (298 K), but our setup allows us to explore temperature variations. Our goals are to refine available line lists for gas-phase water by combining laboratory measurements with quantum chemical calculations, and to reevaluate field measurements. In particular, we will revisit field data from University of Colorado Airborne Multi-AXis Differential Optical Absorption Spectroscopy (CU AMAX-DOAS) instrument during the Tropical Ocean tRoposphere Exchange of Reactive halogen species and Oxygenated VOC (TORERO) campaign. As part of TORERO, comparisons of in situ and remote-sensing measurements of water vapor were performed, and AMAX-DOAS fits exhibited cosmetic residual structures when using HITRAN and HITEMP reference spectra. Our work has the potential to improve trace gas retrievals from many current and planned satellites, e.g. Ozone Monitoring Instrument (OMI), TROPOspheric Monitoring Instrument (TROPOMI), and Tropospheric Emissions: Monitoring of Pollution (TEMPO); and from aircraft-based remote-sensing instruments such as the CU AMAX-DOAS.

The photoabsorbing bands of CO and its isotopologues appearing between 101 and 115 nm provide more than half of its photodissociative potential in the interstellar medium and planetary atmospheres, and are responsible for the well-known fractionation of C and O isotopes due to self-shielding.
An experimental study of this region over several years using the undulator radiation source and vacuum-ultraviolet Fourier-transform spectroscopy facilities at the SOLEIL synchrotron [1] is complete. Line frequencies [2] and oscillator strengths [3], and widths [in prep.] are deduced, and in some cases extrapolated, to provide updated and reliable cross sections over a range of temperatures, including for the rare ¹⁷O isotopologues.

1 N. de Oliveira et al. (2016). The high-resolution absorption spectroscopy branch on the VUV beamline DESIRS at SOLEIL. J. Synchrotron Radiat. 23:887.
2 J.L. Lemaire et al. (2018). Atlas of new and revised high-resolution spectroscopy of six CO isotopologues in the 101-115 nm range. Astron. Astrophys. (accepted)
3 G. Stark et al. (2014). High-resolution oscillator strength measurements of the v=0,1 bands of the B-X, C-X, and E-X systems in five isotopologues of carbon monoxide. Astrophys. J. 788:68

Selected P-and R-branch transitions of the CN A ²Π−X ²Σ⁺ (2-0) band have been recorded at room temperature as a function of argon pressure, using frequency modulation spectroscopy. The experimental line shapes have been successfully fit using a Quadratic Speed-Dependent Voigt (QSDV) model at total pressures ranging from 1 - 160 Torr. The pressure broadening coefficients derived from the QSDV analysis are nearly independent of the rotational quantum state and rotational branch. The pressure-dependence of the line shifts, in contrast, displays a distinctive variation with rotational state and branch, with larger and strongly J-dependent shifts for P-branch lines, compared to smaller and more J-independent shifts for R-branch lines. The pressure-induced shifts provide a challenge to first principles scattering calculations on validated potential energy surfaces. Previously puzzling measurements on fewer lines in the (1-0) band are confirmed and extended by the present measurements.

Dynamic Stark effects, such as Autler-Townes splitting (ATS) and Electromagnetically induced transparency (EIT), has been known to happen in simple atoms or quantum confined systems so far. We have been observed similar power-dependent peak splitting of resonant two-photon ionization (R2PI) spectrum in a polyatomic molecule. In its R2PI spectrum, doublet structures start to appear even at a very weak nanosecond-laser field and show vibronic mode-specificity. Prominent isotope substitution effect indicates that this phenomenon comes from the excited state dynamics.