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Correct pressure broadening is essential for modelling radiative transfer in atmospheres, however data are lacking for the many exotic molecules expected in exoplanetary atmospheres. Here we explore modern machine learning methods to mass produce pressure broadening parameters for a large number of molecules in the ExoMol data base. To this end, state-of-the-art machine learning models are used to fit to existing, empirical air-broadening data from the HITRAN database. A computationally cheap method for large-scale production of pressure broadening parameters is developed, which is shown to be reasonably (69%) accurate for unseen active molecules. This method has been used to augment the previously insufficient ExoMol line broadening diet, providing air-broadening data for all ExoMol molecules, so that the ExoMol database has a full and more accurate treatment of line broadening. Suggestions are made for improved air-broadening parameters for species present in atmospheric databases. [Display omitted] • A machine learning model is trained on the air-broadening data available in the HITRAN database. • A 69% reproduction of the HITRAN data within stated uncertainties is achieved. • Improvements in air broadening data are suggested for some species. • The algorithm is used to populate the ExoMol database. [ABSTRACT FROM AUTHOR]

Room temperature O 2 -broadening coefficients were measured for 16 transitions, from P(23) to R(16), of the ν 4 + ν 5 band of acetylene near 7.5 μm. Absorption spectra of these lines were recorded at room temperature and various pressures, from 20.00 mbar to 66.21 mbar using a tunable diode laser spectrometer. The broadening coefficients were obtained by fitting the Voigt profile as well as the Rautian, the Galatry and the quadratic Speed Dependent Voigt models on the experimental spectra. The results were then compared with some previous studies realized for other bands of acetylene. [ABSTRACT FROM AUTHOR]

Abstract: New absorption cells designed specifically to achieve stable temperatures down to 66K inside the sample compartment of an evacuated Bruker IFS-125HR Fourier transform spectrometer (FTS) were developed at Connecticut College and tested at the Jet Propulsion Laboratory (JPL). The temperature stabilized cryogenic cells with path lengths of 24.29 and 20.38cm were constructed of oxygen free high conductivity (OFHC) copper and fitted with wedged ZnSe windows using vacuum tight indium seals. In operation, the temperature-controlled cooling by a closed-cycle helium refrigerator achieved stability of ±0.01K. The unwanted absorption features arising from cryodeposits on the cell windows at low temperatures were eliminated by building an internal vacuum shroud box around the cell which significantly minimized the growth of cryodeposits. The effects of vibrations from the closed-cycle helium refrigerator on the FTS spectra were characterized. Using this set up, several high-resolution spectra of methane isotopologues broadened with nitrogen were recorded in the 1200–1800cm−1 spectral region at various sample temperatures between 79.5 and 296K. Such data are needed to characterize the temperature dependence of spectral line shapes at low temperatures for remote sensing of outer planets and their moons. Initial analysis of a limited number of spectra in the region of the R(2) manifold of the ν 4 fundamental band of 13CH4 indicated that an empirical power law used for the temperature dependence of the N2-broadened line widths would fail to fit the observed data in the entire temperature range from 80 to 296K; instead, it follows a temperature-dependence similar to that reported by Mondelain et al. . The initial test was very successful proving that a high precision Fourier transform spectrometer with a completely evacuated optical path can be configured for spectroscopic studies at low temperatures relevant to the planetary atmospheres. [Copyright &y& Elsevier]

Abstract: We present semiclassical calculations based on the Robert–Bonamy formalism of collisional broadening coefficients of ammonia by collision with He at room temperature. Calculations have been performed with the accurate ab initio potential of Hodges and Wheatley. The calculated line widths are compared to the available experimental data and a satisfactory agreement is found, although the model contains no adjustable parameters. However the calculations also exaggerate the K dependence of the broadenings, especially for lines with K = J −1 or K = J. [Copyright &y& Elsevier]

Abstract: Using a tunable diode-laser spectrometer, N2-broadening coefficients have been measured for 15 lines in the ν 3 band of C32S2 at room and low temperatures (298, 273.2, 248.2, 223.2, and 198.2K). These lines with J values ranging from 2 to 62 are located in the spectral range 1519–1545cm−1. The collisional widths are obtained by fitting each measured spectral line with a Voigt and a Rautian lineshape model and for a few lines we also used a Galatry model. From these results, we have determined the n parameter of the temperature dependence of each broadening coefficient. A semiclassical calculation of these broadenings has been performed by considering in addition to the main electrostatic quadrupole–quadrupole interaction an anisotropic dispersion contribution leading to satisfactory results at all temperatures and providing the n temperature dependence parameter in good agreement with the experimental determination. [Copyright &y& Elsevier]

Abstract: The CO2-broadening coefficients of 24 P- and R-branch transitions in the ν 4 + ν 5 band of acetylene were measured at room temperature using a diode-laser spectrometer. These lines with J values up to 26, were located in the spectral range 1270 to 1400cm−1. The collisional broadening coefficients were retrieved by fitting the experimental profiles to the Voigt, Rautian, and Galatry lineshape models. Two experimental values for the narrowing coefficient were determined from the spectra and compared with the theoretical narrowing coefficient. The calculations of these broadenings were also performed in the frame of a semiclassical formalism involving a simple intermolecular potential with an adjustable parameter. The theoretical results are in good agreement with the experimental results and reproduce well the J dependence of the broadening coefficients. [Copyright &y& Elsevier]

Abstract: Pressure broadening of phosphine lines by helium and argon at room temperature has been experimentally investigated by high-resolution diode-laser spectroscopy. The broadening coefficients are measured for 38 transitions of PH3 in the Q R branch of the ν 2 band and in the P P and R P branches of the ν 4 band. The recorded lines with J values ranging from 3 to 14 and K from 0 to 10 are located between 1062 and 1094cm−1. The retrieval of the collisional widths is carried out by fitting each spectral line with a Voigt profile, a Rautian profile and a speed-dependent Rautian profile. The latter model provides larger broadening coefficients than the Voigt model. They are also calculated on the basis of a semiclassical model involving the atom–atom Lennard–Jones potential. The theoretical results are in reasonable agreement with the experimental data and reproduce the J and K dependencies of the broadenings. [Copyright &y& Elsevier]

Using a tunable diode-laser spectrometer, we have measured H2-broadening coefficients of PH3 at low temperature (173.2 K) for 27 lines in the QR branch of the ν2 band and in the PP and RP branches of the ν4 band. The recorded lines with J values ranging from 2 to 11 and K from 0 to 9 are located between 1016 and 1093cm−1. The collisional widths are determined by fitting each spectral line with a Voigt profile and a speed-dependent Rautian profile which provides slightly larger broadening coefficients than the Voigt model. These coefficients have also been calculated on the basis of a semiclassical model of interacting linear molecules by considering an atom–atom Lennard–Jones potential in addition to the weak electrostatic contributions. Except for three QR(J,K) lines, where K=J, the calculated broadening coefficients are in good agreement with the experimental data. By comparing the results obtained at room and low temperatures, the temperature dependence of linewidths has been determined both theoretically and experimentally. [Copyright &y& Elsevier]

The spectra of the gaseous mixtures CH4–He and CH4–Ar were obtained in the spectral region 1400–1750 cm-1 with a resolution up to 0.003 cm-1. Helium and argon pressure broadenings for the vibration–rotation lines of the ν2 band of CH4 have been estimated at room temperature for some lines in the P, Q, and R branches. These values were also calculated using the theoretical approach developed by Robert and Bonamy, extended to the case of tetrahedral molecules. The helium data have been found to be in a satisfactory agreement whereas a divergence of calculated and measured broadening coefficients has been evidenced in the case of argon. Simulations of the ν2 band shapes of methane perturbed by helium have also been performed. [Copyright &y& Elsevier]

Fourier transform spectroscopy has been used to record the absorption line shapes for rotational transitions in the Δv=0, b1Σg+←X3Σg− band of O2 in the presence of the noble gases. We determined angular momentum dependent collision broadening coefficients for each noble gas using the Lorentzian widths that were extracted from Voigt fits to the line shapes. The values were largest at low J′′ and, on average, decreased by 39% for the highest J′′ states. Ten “scaling/fitting laws” designed for the analysis of rotational energy transfer data were fit to the broadening cross-section data. Both the angular momentum based ECS-P and the energy based EPGL-0 descriptions accurately represented the data across the entire range of rotational levels probed in this study. Pressure-induced line shifting was also studied in the O2–Xe collision system. The pressure shifts were all negative. [Copyright &y& Elsevier]

For the purpose of atmospheric applications, we have measured N2- and O2-induced broadenings and shapes of rotational lines of N2O in the 235–350 K temperature range, precisely the J=8←7, J=22←21, and J=23←22 lines, located near 201, 552, and 577 GHz, respectively. The analysis of experimental lineshapes shows up significant deviations from the Voigt profile, which are characteristic of line narrowing processes. In a first step, the Voigt profile was considered for the determination of pressure broadening parameters and of their temperature dependencies. Results are in good agreement with the dependence from rotational quantum number previously observed for other rotational and rovibrational lines. They are well explained by calculations based on a semiclassical formalism that includes the atom–atom Lennard-Jones potential in addition to electrostatic interactions up to hexadecapolar contributions. In a second step, observed lineshapes were analyzed by using the Galatry profile and a speed-dependent Voigt profile. The nonlinear pressure behavior observed for the diffusion rate β involved in the Galatry profile leads to rule out the possible role of velocity/speed changing collisions, and to infer that discrepancies from the Voigt profile result from the dependence of relaxation rates on molecular speeds. This interpretation is supported by the comparison of optical and kinetic radii and confirmed by theoretical calculations of relaxation rates. Finally, it can be claimed that, for the N2O–N2 and N2O–O2 systems, deviations from the Voigt profile are explained by a speed-dependent Voigt profile. [Copyright &y& Elsevier]

Low pressure measurements of broadening parameters of the 118.75 GHz fine structure line of oxygen molecule have been made by a BWO-based spectrometer with acoustic detector (RAD) at room temperature. Pressure broadening parameters were obtained for the buffer gases O2, N2, Ne, He, Ar, H2O, CO2, and CO and have the following values 2.23 ± 0.01, 2.245 ± 0.02, 1.375 ± 0.02, 1.62 ± 0.03, 2.005 ± 0.02, 2.52 ± 0.04, 2.66 ± 0.08, and 2.31 ± 0.05 MHz/Torr, respectively. Measured central frequency is 118 750.340 ± 0.007 MHz. The central frequencies and broadenings by O2 and N2 of fine structure lines 1+, 5−, 7+, 11+, and 15− belonging to the 60-GHz band are also measured. Comparison of previous and recent data on electronic, rotational, and fine structure lines broadenings reveals their close values (within 10%) and dependencies on corresponding rotational quantum numbers for these different oxygen spectra stretching from millimeter through submillimeter up to the optical bands. Such similarity could be used for estimation of the broadenings of not measured yet oxygen lines. [Copyright &y& Elsevier]

We present measurements of Ar-broadening parameters for the R(0) and R(7) lines in the fundamental band of 13CO at eight temperatures from 80 to 297 K. The broadening parameters are determined by simultaneous least-squares fitting of spectra recorded using a frequency stabilized diode laser spectrometer. The comparison of the broadening parameter values for R(7) derived at room temperature and different pressures from different line profiles shows that an empirical line profile, which takes into account narrowing effects (Dicke narrowing and absorber speed dependence) but neglects any correlation between collisions, is able to describe the observed lines with constant values of the narrowing and broadening parameters over a 10–500 Torr pressure range. Starting from a recent ab initio potential energy surface, theoretical thermally averaged close coupling values of the Ar broadening parameter are calculated for the same temperatures. The comparison between experimental and calculated values shows an overall agreement of 1.5%. [Copyright &y& Elsevier]

We report on linewidth measurements on the J=24K,11−23K′,10 and J=38K,33−37K′,32 millimeter wave transitions in the ground vibrational state of nitric acid, located near 470.23 and 544.36 GHz, respectively. Experiments were performed with N2 and O2 as perturber molecules, in the 240–350 K temperature range by using a video-type spectrometer. The foreign-gas broadening parameters and their temperature dependence coefficients were determined using the Voigt profile, no narrowing effect being observed. In order to check the reliability of reported values, we carried out measurements on the J=14K,12−13K′,11 transition located near 206.6 GHz, previously observed in two other laboratories. For this last line all the reported values are consistent themselves within one claimed standard deviation. [Copyright &y& Elsevier]