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We are now on a clear trajectory for improvements in exoplanet observationsthat will revolutionize our ability to characterize their atmosphericstructure, composition, and circulation, from gas giants to rocky planets.However, exoplanet atmospheric models capable of interpreting the upcomingobservations are often limited by insufficiencies in the laboratory andtheoretical data that serve as critical inputs to atmospheric physical andchemical tools. Here we provide an up-to-date and condensed description ofareas where laboratory and/or ab initio investigations could fill critical gapsin our ability to model exoplanet atmospheric opacities, clouds, and chemistry,building off a larger 2016 white paper, and endorsed by the NAS ExoplanetScience Strategy report. Now is the ideal time for progress in these areas, butthis progress requires better access to, understanding of, and training in theproduction of spectroscopic data as well as a better insight into chemicalreaction kinetics both thermal and radiation-induced at a broad range oftemperatures. Given that most published efforts have emphasized relativelyEarth-like conditions, we can expect significant and enlightening discoveriesas emphasis moves to the exotic atmospheres of exoplanets.

The sulfur dimer (S2) is an important molecular constituent in cometary atmospheres and volcanic plumes on Jupiter's moon Io. It is also expected to play an important role in the photochemistry of exoplanets. The ultraviolet (UV) spectrum of S2 contains transitions between vibronic levels above and below the dissociation limit, giving rise to a distinctive spectral signature. Using spectroscopic information from the literature, and the spectral simulation program PGOPHER, a UV line list of S2 is provided. This line list includes the primary |$B\, ^{3}\Sigma ^{-}_{u}-X\, ^{3}\Sigma ^{-}_{g}$| (v ′ = 0–27, v ″ = 0–10) electronic transition, where vibrational bands with v ′≥10 are pre-dissociated. Intensities have been calculated from existing experimental and theoretical oscillator strengths, and semi-empirical strengths for the pre-dissociated bands of S2 have been derived from comparisons with experimental cross-sections. The S2 line list also includes the |$B^{\prime \prime }\, ^{3}\Pi _{u}-X\, ^{3}\Sigma ^{-}_{g}$| (v ′ = 0–19, v ″ = 0–10) vibronic bands due to the strong interaction with the B state. In summary, we present the new HITRAN-formatted S2 line list and its validation against existing laboratory spectra. The extensive line list covers the spectral range 21 700–41 300 cm−1 (∼242–461 nm) and can be used for modelling both absorption and emission. [ABSTRACT FROM AUTHOR]

Calculating dipole moments with high-order basis sets is generally only possible for the light molecules, such as water. A simple, yet highly effective strategy of obtaining high-order dipoles with small, computationally less expensive basis sets is described. Using the finite field method for computing dipoles, energies calculated with small basis sets can be extrapolated to produce dipoles that are comparable to those obtained in high order calculations. The method reduces computational resources by approximately 50% (allowing the calculation of reliable dipole moments for larger molecules) and simultaneously improves the agreement with experimentally measured infrared transition intensities. For atmospherically important molecules, which are typically too large to consider the use of large basis sets, this procedure will provide the necessary means of improving calculated spectral intensities by several percent. [ABSTRACT FROM AUTHOR]

Water vapor absorption in the near-ultraviolet region is essential to describe the energy budget of Earth, but little spectroscopic information is available since it is a challenging spectral region for both experimental and theoretical studies. A continuous-wave cavity ring-down spectroscopic experiment was built to record absorption lines of water vapor around 415 nm. With a precision of 4×10-10 cm -1 , 40 rovibrational transitions of H 216 O were observed in this work, and 27 of them were assigned to the (224), (205), (710), (304), (093), (125) and (531) vibrational bands. A comparison of line positions and intensities determined in this work to the most recent HITRAN database is presented. Water vapor absorption cross-sections near 415 nm were calculated based on our measurements, which vary between 1×10-26 and 5×10-26 cm 2 molec. -1. These data will also significantly impact the spectroscopy detection of trace gas species in the near-UV region. [ABSTRACT FROM AUTHOR]

An optimal estimation-based algorithm is developed to retrieve the number density of excited oxygen (O 2) molecules that generate airglow emissions near 0.76 µ m (b1Σg+ or A band) and 1.27 µ m (a1Δg or 1Δ band) in the upper atmosphere. Both oxygen bands are important for the remote sensing of greenhouse gases. The algorithm is applied to the limb spectra observed by the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) instrument in both the nominal (tangent heights below ∼ 90 km) and mesosphere–lower thermosphere (MLT) modes (tangent heights spanning 50–150 km). The number densities of emitting O 2 in the a1Δg band are retrieved in an altitude range of 25–100 km near-daily in 2010, providing a climatology of O 2 a1Δg -band airglow emission. This climatology will help disentangle the airglow from backscattered light in nadir remote sensing of the a1Δg band. The global monthly distributions of the vertical column density of emitting O 2 in a1Δg state show mainly latitudinal dependence without other discernible geographical patterns. Temperature profiles are retrieved simultaneously from the spectral shapes of the a1Δg -band airglow emission in the nominal limb mode (valid altitude range of 40–100 km) and from both a1Δg - and b1Σg+ -band airglow emissions in the MLT mode (valid range of 60–105 km). The temperature retrievals from both airglow bands are consistent internally and in agreement with independent observations from the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), with the absolute mean bias near or below 5 K and root mean squared error (RMSE) near or below 10 K. The retrieved emitting O 2 number density and temperature provide a unique dataset for the remote sensing of greenhouse gases and constraining the chemical and physical processes in the upper atmosphere. [ABSTRACT FROM AUTHOR]

The latest version of the HITRAN molecular spectroscopic data base, HITRAN2020, has recently been released featuring many updates, including line-by-line broadening parameters (and their temperature dependence) appropriate for the dominant constituents of planetary atmospheres. In this work, line shape codes suitable for calculating microwave spectra have been implemented within the HITRAN Application Programming Interface (HAPI). These new additions allow for spectroscopic calculations of microwave absorbing species pertinent to current and future studies of the atmospheres of Jupiter and Venus, and more generally for the atmospheres of gas giants and rocky planets. The inversion spectrum of the NH3 molecule broadened by H2, He, and H2O dominates the microwave region of Jupiter, whereas for Venus, accurate spectroscopic data of SO2 broadened by CO2 are necessary in order to determine its significance, if any, on the reported detection of PH3 in the Venusian upper cloud deck. Comparisons have been made to available microwave laboratory opacities and the following results illustrate that HITRAN data can be used in conjunction with HAPI to reproduce the existing experimental measurements and provide reliable calculation of planetary opacities. Users should be mindful regarding selection of appropriate parameters in HITRAN and selecting suitable line shape functions in HAPI, depending on the spectral region, target molecular species, as well as ambient chemical and thermodynamic conditions. [ABSTRACT FROM AUTHOR]

We study the anomalies in the distributions of intensities of transitions in the purely rotational bands and the rotational branches of the vibrational bands within the unperturbed ground electronic states in spectra of diatomic molecules. While normally these distributions follow smooth patterns, sudden drops in intensity values are often observed. We analyze the origin of these anomalies in HF, DF, and CO and find that they are predominantly associated with specific forms of the dipole-moment functions (DMFs). The rotational transitions at which these anomalies occur and their severity are very sensitive to these forms, which makes them a promising tool for refining the empirical DMFs. [ABSTRACT FROM AUTHOR]

Water vapor absorption in the near-ultraviolet region is essential to describe the energy budget of Earth, but little spectroscopic information is available since it is a challenging spectral region for both experimental and theoretical studies. A continuous-wave cavity ring-down spectroscopic experiment was built to record absorption lines of water vapor around 415 nm. With a minimum detectable absorption coefficient of 4×10-10 cm-1, 40 rovibrational transitions of H162O were observed in this work, and 27 of them were assigned to the (224), (205), (710), (304), (093), (125), and (531) vibrational bands. A comparison of line positions and intensities determined in this work to the most recent HITRAN database is presented. Water vapor absorption cross-sections near 415 nm were calculated based on our measurements, which vary between 1×10-26 and 5×10-26 cm²/molecule. These data will also significantly impact the spectroscopy detection of trace gas species in the near-UV region. [ABSTRACT FROM AUTHOR]