The partially correlated quadratic, speed-dependent hard-collision profile (pCqSDHCP), for simplicity referred to as the Hartmann-Tran profile (HTP), Tran, H., N. Ngo, and J.-M. Hartmann, Journal of Quantitative Spectroscopy and Radiative Transfer 2013. 129: p. 89-100.as been recommended as a generalized lineshape for high resolution spectroscopy.Tennyson, et al., Pure Appl. Chem. 2014, 86: p. 1931-1943.he HTP parameterizes complex collisional effects such as Dicke narrowing, speed dependent narrowing, and correlations between velocity-changing and dephasing collisions, while also simplifying to simpler profiles that are widely used, such as the Voigt profile. As advanced lineshape profiles are adopted by more researchers, it is important to understand the limitations that data quality has on the ability to retrieve physically meaningful parameters using sophisticated lineshapes that are fit to spectra of finite signal-to-noise ratio. In this work, spectra were simulated using the HITRAN Application Programming Interface (HAPI)Kochanov, R.V., et al., Journal of Quantitative Spectroscopy and Radiative Transfer 2016. 177: p. 15-30.cross a full range of line parameters.Tran, H., N. Ngo, and J.-M. Hartmann, Journal of Quantitative Spectroscopy and Radiative Transfer 2013. 129: p. 199-203.imulated spectra were evaluated to quantify the precision with which fitted lineshape parameters can be determined at a given signal-to-noise ratio, focusing on the numerical correlation between the retrieved Dicke narrowing frequency and the velocity-changing and dephasing collisions correlation parameter. Tran, H., N. Ngo, and J.-M. Hartmann, Journal of Quantitative Spectroscopy and Radiative Transfer 2013. 129: p. 89-100.h Tennyson, et al., Pure Appl. Chem. 2014, 86: p. 1931-1943.T Kochanov, R.V., et al., Journal of Quantitative Spectroscopy and Radiative Transfer 2016. 177: p. 15-30.a Tran, H., N. Ngo, and J.-M. Hartmann, Journal of Quantitative Spectroscopy and Radiative Transfer 2013. 129: p. 199-203.S

We present high signal-to-noise ratio, mode-by-mode cavity ring-down spectroscopy (CRDS) line shape measurements of air-broadened transitions in the 30013 → 0001 band of ¹²C¹⁶O₂ located near λ = 1.6 μm. Absorption spectra were acquired from (230-290) K with a variable-temperature spectrometer developed in the framework of the NASA Orbiting Carbon Observatory-2 Mission to improve our understanding of carbon dioxide and oxygen line shape parameters. This system comprises a monolithic, thermally stabilized two-mirror, optical resonator exhibiting a mode stability of 200 kHz and a minimum detectable absorption coefficient of 10⁻¹¹ cm⁻¹. Observed spectra were modeled the using the recently recommended Hartmann-Tran line profile (HTP) Tennyson, et al., Pure Appl. Chem. 86, (2014) 1931and several of its limiting cases) which includes the effects of Dicke narrowing, speed dependent broadening, correlation between velocity- and phase-changing collisions and first-order line mixing effects. At fixed temperature, line shape parameters were determined by constrained multispectrum fitting of spectra acquired over the pressure range (30 - 300) Torr. For each transition considered, analysis of the temperature dependence of the fitted line shape parameters yielded the pressure-broadening temperature exponent and speed dependence parameter, where the latter quantity was found to be in good agreement with theoretical values consistent with the HTP model.

---

Footnotes:

Tennyson, et al., Pure Appl. Chem. 86, (2014) 1931(

To improve the understanding of temperature-dependence laws of spectral line shape parameters, spectra of the ν₃ rovibrational band of CO₂ perturbed by 10, 30, 100, 300 and 1000 mbar of N₂ were measured at nine temperatures between 190 K and 330 K using a 22 cm long single-pass absorption cell in a Bruker IFS125 HR Fourier Transform spectrometer. The spectra were fitted employing a quadratic speed-dependent hard collision model in the Hartmann-Tran implementationNgo et al. JQSRT 29, 89-100 (2013); JQSRT 134, 105 (2014).Tran et al. JQSRT 129, 199-203 (2013); JQSRT 134, 104 (2014). extended to account for line mixing in the Rosenkranz approximation by means of a multispectrum fitting approach developed at DLRLoos et al., 2014; http://doi.org/10.5281/zenodo.11156. This enables high accuracy parameter retrievals to reproduce the spectra down to noise level and we will present the behavior of line widths, shifts, speed-dependence-, collisional narrowing- and line mixing-parameters over this 140 K temperature range.

---

Footnotes:

Ngo et al. JQSRT 29, 89-100 (2013); JQSRT 134, 105 (2014).

---

Footnotes:

Loos et al., 2014; http://doi.org/10.5281/zenodo.11156..

We have measured several carbon monoxide transitions in the (3→0) and (4→1) band using frequency stabilized cavity ringdown spectroscopy (FS-CRDS). The measured transitions are compared to the line strength values in HITRAN 2012 [1], those determined by Wojtewitz et al [2], and to theoretical calculations. The cavity length is actively locked to an iodine stabilized HeNe laser, providing long term frequency stability of 10 kHz and is linked to a self-referenced, octave-spanning frequency comb. The temperature of the optical cavity is actively regulated at the mK level, and the pressure measurements are SI-traceable. The sample is a NIST calibrated reference mixture of 11.98575(95)% CO in N₂. The absorption spectra are modeled using the Hartmann-Tran profile (HTP). The SNR in these spectra may exceed 10,000:1, which necessitates including the effects of speed dependence, collisional narrowing, and correlation between velocity-changing and dephasing collisions. The relative uncertainties of the line strengths calculated in this study are better than 0.1%. There are systematic differences on the 1% level for ¹²CO against both HITRAN [1] and the previous work by Wojtewitz et al [2]. The measurement uncertainties are nearly an order of magnitude lower than previous results. Additionally, the relative uncertainties in the integrated areas of selected ¹²CO and ¹³CO transitions are less than 0.006% and 0.02%, respectively, providing an excellent test case for determination of isotope ratios by direct use of theoretical line intensity calculations. [1] Wojtewicz, S., et al., J Quant Spect and Rad Trans,2013. 130: p.191-200. [2]Rothman, L.S., et al., Journal of Quant Spect and Rad Trans, 2013. 130: p. 4-50.

The phenomenon of collisional transfer of intensity due to line mixing has an increasing importance for atmospheric monitoring. From a theoretical point of view, all relevant information about the collisional processes is contained in the relaxation matrix W where the diagonal elements give half-widths and shifts, and the off-diagonal elements correspond to line interferences. For simple systems such as diatom-atom and diatom-diatom, fully quantum calculations are feasible, but become unrealistic for more complex systems. Meanwhile, the semi-classical Robert-Bonamy (RB) formalism widely used to calculate half-widths and shifts completely fails in calculating the off-diagonal elements of W resulting from applying the isolated line approximation. Recently, we have developed a new semi-classical formalism without this approximation that enables one not only to reduce uncertainties for calculated half-widths and shifts, but also to calculate the off-diagonal elements. This implies that we can address line mixing based on interaction potentials between molecular absorber and molecular perturber. In the present study, we have applied this method to calculate the relaxation matrices for self-broadened NH₃ lines in the parallel pure-rotational, ν₁, ν₂, and 2ν₂ bands and also in the perpendicular ν₄ band. Our studies have exhibited a significant off-diagonality of W in the pure-rotational, ν₁, and ν₄ bands. For the ν₂ and 2ν₂ bands, the off-diagonality is much less and even becomes completely absent. Given the fact that the inversion doublet splitting is the main source responsible for the off-diagonality of W and its value in these bands dramatically increases from less than 1 cm⁻¹, to 36 cm⁻¹, and further to 284 cm⁻¹, it is easy to understand these results. By comparing with half-widths derived from the RB formalism, our values in the pure-rotational, ν₁, and ν₄ bands are significantly reduced and match measurements very well. We have also compare calculated off-diagonal elements of W and Rosenkranz line mixing coefficients with measured results. In addition, we have compared the calculated profiles, including line mixing effects with the observed ones in various cases: a good agreement is obtained in the ^PP doublets of the ν₄ band as well as in the Q branch and the R(3,k) manifold in the ν₁ band. For some other measurements reported in literature, very large discrepancies (up to two orders) have been found and our comments on these measurements are presented.

[10]r0.32 Figure Saturation dip spectra of acetylene in the v₁ + v₃ band have been obtained for rotational lines with J = 31−37 inclusive, using a diode laser referenced to a frequency comb. The estimated accuracy and precision of the measurements is better than 10 kHz in 194 THz. Data were obtained as a function of sample pressure to investigate the broadening of the saturation features. The observed line shapes are well modeled by convolution of a fixed Gaussian transit-time and varying Lorentzian lifetime broadening, i.e. a Voigt-type profile. The lines exhibit a significantly larger collisional (lifetime) broadening than has been measured in conventional Doppler and pressure-broadened samples at ambient temperatures. The figure shows the fitted Lorentzian width versus sample pressure for P(31). The slope of this plot gives the pressure broadening coefficient, γ_self = 9.35(13) MHz/mbar. For comparison, the coefficient derived from conventional Doppler and pressure broadened spectra for this transition is 2.7 MHz/mbarJ. Molec. Spectrosc. 209, 216-227 (2001) and J. Quant. Spectrosc. Rad. Transf. 76, 237-267 (2003) The sub-Doppler broadening coefficients are all significantly larger than the conventionally measured ones, due to the increased importance of velocity-changing collisions. The measurements therefore give information on the balance between hard phase- or state-changing and large cross-section velocity-changing collisions. Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-SC0012704 with the U.S. Department of Energy, Office of Science, and supported by its Division of Chemical Sciences, Geosciences and Biosciences within the Office of Basic Energy Sciences.

---

Footnotes:

J. Molec. Spectrosc. 209, 216-227 (2001) and J. Quant. Spectrosc. Rad. Transf. 76, 237-267 (2003).

We have combined experimental and theoretical approaches to the competition between elastic and inelastic collisions of CN radicals with Ar, and how this competition influences time-resolved saturation spectra. Experimentally, we have measured transient, two-color sub-Doppler saturation spectra of CN radicals with an amplitude chopped saturation laser tuned to selected Doppler offsets within rotational lines of the A-X (2-0) band, while scanning a frequency modulated probe laser across the hyperfine-resolved saturation features of corresponding rotational lines of the A-X (1-0) band. A steady-state depletion spectrum includes off-resonant contributions ascribed to velocity diffusion, and the saturation recovery rates depend on the sub-Doppler detuning. The experimental results are compared with Monte Carlo solutions to the Boltzmann equation for the collisional evolution of the velocity distributions of CN radicals, combined with a pressure-dependent and speed-dependent lifetime broadening. Velocity changing collisions are included by appropriately sampling the energy resolved differential cross sections for elastic scattering of selected rotational states of CN (X). The velocity space diffusion of Doppler tagged molecules proceeds through a series of small-angle scattering events, eventually terminating in an inelastic collision that removes the molecule from the coherently driven ensemble of interest. Collision energy-dependent total cross sections and differential cross sections for elastic scattering of selected CN rotational states with Ar were computed with Hibridon quantum scattering calculations, and used for sampling in the Monte Carlo modeling.
Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-SC0012704 with the U.S. Department of Energy, Office of Science, and supported by its Division of Chemical Sciences, Geosciences and Biosciences within the Office of Basic Energy Sciences.

Multispectrum analysis can be a challenge for a variety of reasons. It can be computationally intensive to fit a proper line shape model especially for high resolution experimental data. Band-wide analyses including many transitions along with interactions, across many pressures and temperatures are essential to accurately model, for example, atmospherically relevant systems. Labfit is a fast multispectrum analysis program originally developed by D. Chris Benner with a text-based interface. More recently at JPL a graphical user interface was developed with the goal of increasing the ease of use but also the number of potential users. The HTP lineshape model has been added to Labfit keeping it up-to-date with community standards. Recent analyses using labfit will be shown to demonstrate its ability to competently handle large experimental datasets, including high order lineshape effects, that are otherwise unmanageable.

Using broadband, high resolution dual frequency comb spectroscopy, we test the power law temperature scaling relationship with Voigt, Rautian, and quadratic speed dependent Voigt profiles over a temperature range of 296-1300K for pure water vapor. The instrument covers the spectral range from 6800 cm⁻¹ to 7200 cm⁻¹ and samples the (101)-(000), (200)-(000), (021)-(000), (111)-(010), (210)-(010), and (031)-(010) vibrational bands of water. The data is sampled with a point spacing of 0.0033 cm⁻¹ and absolute frequency accuracy of < 3.34e-6 cm⁻¹. This region is of interest for detection and quantification of hot water and gas temperature within coal gasifiers and other high temperature systems. In order to extract water concentration and temperature, an extended range of lineshape parameters are needed. Lineshape parameters for pure and argon broadened water are obtained for 278 transitions using the multispectral fitting program Labfit, including self-broadening coefficients, power law temperature scaling exponents, and speed dependence coefficients. The extended temperature range of the data provides valuable insight into the application of the speed-dependence corrections of the line profiles, which are shown to have more reasonable line broadening temperature dependencies.

Molecular oxygen (O₂) has a well-known and uniform molar fraction within the Earth’s atmosphere. Consequently, the O₂ A-band is commonly used in satellite and remote sensing measurements (GOSAT, OCO-2, TCCON) to determine the surface pressure-pathlength product for transmittance measurements that involve light propagation through the atmospheric column. For these missions, physics-based spectroscopic models and experimentally determined line-by-line parameters are used to predict the temperature- and pressure-dependence of the absorption cross-section as a function of wave number, pressure, temperature and water vapor concentration. At present, there remain airmass-dependent biases in retrievals of CO₂ which are linked to limitations in existing models of line mixing (LM) and collisional induced absorption (CIA) Long D.A and J.T. Hodges, J. Geophys. Res. 2012, 117: p. D12309. In order to better quantify these effects, we measured O₂ A-band spectra with a frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) system. Because of the high molar fraction of O₂ in air samples, line cores and near wings of the dominant absorption transitions are heavily saturated, which makes it impossible to obtain continuous FS-CRDS spectra over the entire range of optical depth. Here, we focused on LM and CIA effects which dominate the valleys between strongly absorbing transitions. To this end, the FS-CRDS system employs a thresholding mechanism that avoids the optically thick regions and scans over the entire O₂ A-band and beyond the band head region. This approach provides high signal-to-noise ratio spectra that can be fit to yield LM and CIA parameters. These results are intended to provide strong constraints on multispectrum fits of continuous and broadband Fourier-transform-spectroscopy based O₂ A-band spectra.

---

Footnotes:

Long D.A and J.T. Hodges, J. Geophys. Res. 2012, 117: p. D12309..

Oxygen dimol is transiently generated when two oxygen molecules collide. At this short period, the electron clouds of molecules are distorted and some forbidden transition electronic transitions become partially allowed. This transition is called CIA (Collision-induced absorption). There are several CIA bands appearing in the spectral region from UV to near IR. Absorption of solar radiation by oxygen dimol is a small but significant part of the total budget of incoming shortwave radiation. However, a theory predicting the lineshape of CIA is still under developing. In this study, we measured CIA band around 630 nm that is assigned to optical transition, a¹∆_g(v=0):a¹∆_g(v=0)-X³Σ_g⁻(v=0):X³Σ_g⁻(v=0) of oxygen dimol. CRDS(Cavity Ring-down Spectroscopy) was employed to measure weak absorption CIA band of oxygen. Laser beam around 630 nm was generated by a dye laser that was pumped by a YAG Laser. Multiple reflection of the probe light was performed within a vacuum chamber that was equipped with two high reflective mirrors. We discuss the measured line shape of CIA on the basis of collision pair model.

Collision-induced absorption of O₂−O₂ and O₂−N₂ pairs is observed in remote sensing of the Earth's atmosphere, and absorption by O₂−O₂ pairs has been put forward as a biomarker to be observed in exoplanetary transit spectra. The relevant electronic transitions, X ³Σ_g⁻ → a ¹∆_g and X ³Σ_g⁻ → b ¹Σ_g⁺, are electric-dipole forbidden by both spin and spatial selection rules, such that collision-induced absorption represents an important contribution to the absorption. We present an ab initio study of collision-induced absorption for these electronic transitions using quantum-mechanical scattering calculations. Two mechanisms for breaking the spin-symmetry are taken into account: intramolecular spin-orbit coupling and intermolecular exchange interactions. We find and explain qualitative differences in the line shape and temperature dependence of the absorption due to these mechanisms. The contributions of these mechanisms furthermore explain qualitative differences between the atmospherically relevant O₂−O₂ and O₂−N₂ systems. Reasonable agreement with experimental data is obtained for various near-infrared transitions in both systems.