Collision-induced Absorption Research Articles

New experimental measurements of the Collision Induced Absorptions of H2-H2 and H2-He in the 3600-5500 cm−1 spectral range from 120 to 500 K

The Collision-Induced Absorption (CIA) fundamental band of H2 has been studied in the 3600–5500 cm−1 spectral range for temperatures ranging from 120 to 500 K for both a pure H2 gas and a H2-He mixture. We used a simulation chamber called PASSxS (Planetary Atmosphere System Simulation x Spectroscopy) developed at INAF/ISAC which contains a Multi-Pass cell interfaced with a Fourier Spectrometer, aligned to reach an optical path of 3.28 m. The H2-H2 and H2-He binary absorption coefficients (BACs) have been derived for seven temperatures in the chosen range and provided in tabular form, including the unexplored high-temperature range above 300 K. We also calculated the integral of the H2-H2 and H2-He experimental BACs in the reduced 4000–5000 cm−1 spectral range, finding a linear trend with temperature in both cases. The integrals have also been computed with larger uncertainties for the whole band, in the total 3600–5500 cm−1 spectral range including the band's wings, partially affected by the water vapor absorption. The integrals calculated over the whole and reduced spectral ranges are collected in tables. In addition, we performed measurements with a H2-He mix for different mixing ratios to explore possible deviations from the linear combination of the BACs. The experimental BACs have been shown in comparison with Abel and Borysow's ab initio models for a temperature of about 400 K, resulting in a good agreement over almost the whole spectral range, with a maximum deviation around the main peak of the band. Data and models also show a good agreement in the linear trend of the integrated BACs with temperature, apart from the H2-H2 Borysow's BACs, which follow a quadratic trend. Finally, we resolved all the interference dips, which were not taken into account by the existing theoretical models.

Collision induced absorption spectra of N2 and CH4

Collision-induced absorption (CIA) spectra of nitrogen and methane are needed to model radiative transfer in atmospheres of objects such as Saturn's moon, Titan. We present rare, extremely low temperature Fourier transform infrared spectra of nitrogen and methane rototranslation CIA in the range of 40–600 cm−1. The peak CIA of methane was found to be 2.05×10−5 cm−1/amagat2 at 123.8 K while nitrogen had a peak value of 1.65×10−5 cm−1/amagat2 at an average temperature of 102.5 K. These measurements were made possible by the unique instrumentation available at the Far-IR beamline at the Canadian Light Source.

A novel physics-based cloud retrieval algorithm based on neural networks (CRANN) from hyperspectral measurements in the O2-O2 band

Clouds play a crucial role in the Earth's climate system and their properties can be detected by hyperspectral measurements from space. With the increasing spectral resolution, traditional retrieval methods based on look-up tables (LUT) and optimal estimation are limited in both efficiency and accuracy compared with machine learning methods. However, the machine learning techniques used to establish the relationships between spectral measurements and cloud properties often lack physical explainability and universality. Additionally, traditional physical retrieval methods based on oxygen A-band are not applicable to instruments without the O2-A band like the ozone monitoring instrument (OMI). Therefore, we have proposed a novel physics-based deep neural networks (DNN) retrieval method––the cloud retrieval algorithm based on neural networks (CRANN)––which incorporates a deep neural network model with radiative transfer model to retrieve cloud fraction and cloud-top pressure from the oxygen–oxygen collision-induced (O4) absorption band. Validation using simulated test data supported the superior accuracy of CRANN, with the correlation coefficients for cloud fraction and cloud-top pressure are 0.989 and 0.993, respectively, whereas the correlation coefficients for cloud fraction and cloud-top pressure of the LUT method are 0.928 and 0.865, respectively. In comparison with the OMCLDO2 cloud product from the OMI, the CRANN results retrieved from OMI observations exhibit substantial consistency, boasting correlation coefficients surpassing 0.95 for cloud fraction and 0.83 for cloud pressure. As compared with the tropospheric monitoring instrument (TROPOMI) official products, the CRANN retrieval results from TROPOMI exhibit a high level of consistency with correlation coefficients exceeding 0.8 for cloud fraction and 0.73 for cloud pressure. Additionally, a promising agreement is observed between the CRANN retrievals from TROPOMI and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data, yielding RMSEs of 127.3, 134.6 and 106.4 hPa for the validation dataset, respectively.

Accurate neural-network-based fitting of full-dimensional N2 –Ar and N2–CH4 two-body potential energy surfaces aimed at spectral simulations

We describe the development of machine-learned (ML) potentials for flexible, weakly interacting monomers. A recently suggested permutationally invariant polynomial neural network (PIP-NN) approach is utilised to represent the full-dimensional two-body component of the molecular pair energy. To ensure the asymptotic zero-interaction limit, a tailored subset of the full invariant polynomial basis set is utilised, and their variables are modified to achieve a better fit of the correct asymptotic behaviour at a long range. This new technique is used to build full-dimensional potentials for the two-body N 2 –Ar and N 2 –CH 4 interactions by fitting databases of ab initio energies calculated at the coupled-cluster level of theory. The second virial coefficient, fully accounting for molecular flexibility, is then calculated within the classical framework using the obtained PIP-NN potential surfaces. A trajectory-based simulation of the N 2 –Ar collision-induced absorption is conducted, covering both the far- and mid-infrared ranges.

First-principles approach to calculating collision-induced absorption for H2–H2 pairs for the rotational, fundamental, and first overtone bands at 300K

A absorção na banda roto-translacional de H2 -H2 é relevante para as atmosferas de corpos celestes e planetas externos, caracterizados por temperaturas elevadas, promovendo uma excitação roto-vibracional significativa. Uma análise comparativa do espectro de absorção H2 -H2 com medições laboratoriais a 300K revela uma concordância consistente em toda a faixa de frequência (0 - 10000 cm-1). O espectro é principalmente influenciado pelos componentes do dipolo induzido pelo quadrupolo (λ1 λ2 ΛL), com as contribuições mais significativas provenientes de 0223 e 2023. Estes componentes, especialmente 0223 e 2023, desempenham um papel dominante. Além de 3000 cm-1, componentes adicionais como a interação quadrupolar (λ1 λ2 ΛL = 2233) e outros (λ1 λ2 ΛL = 0221, 0443, 4043, 0445 e 4045) contribuem para a absorção total.

The collision-induced absorption of H2 near 1.20 µm: Subatmospheric measurements and validation tests of calculations

The weak binary collision-induced absorption (CIA) of molecular hydrogen is measured at room temperature in the first overtone region near 1.20 µm. Binary absorption coefficients are derived by cavity ring down spectroscopy (CRDS) at 28 selected spectral points sampling the (2–0) band between 7974 and 8650 cm−1. While all previous studies used high density samples, the sensitivity of the CRDS method allowed deriving accurate CIA by using pressure ramps of pure H2 limited to a maximum pressure of 1 atm. After subtraction of the Rayleigh contribution, a purely quadratic pressure dependence is obtained for the absorption coefficient at each measurement point and the CIA binary coefficients are derived with a 1.5 % accuracy. The comparison to theoretical values widely used for astronomical applications shows deviations values between 5 and 25 %.

Data availability and requirements relevant for the Ariel space mission and other exoplanet atmosphere applications

ABSTRACT The goal of this white paper is to provide a snapshot of the data availability and data needs primarily for the Ariel space mission, but also for related atmospheric studies of exoplanets and cool stars. It covers the following data-related topics: molecular and atomic line lists, line profiles, computed cross-sections and opacities, collision-induced absorption and other continuum data, optical properties of aerosols and surfaces, atmospheric chemistry, UV photodissociation and photoabsorption cross-sections, and standards in the description and format of such data. These data aspects are discussed by addressing the following questions for each topic, based on the experience of the ‘data-provider’ and ‘data-user’ communities: (1) what are the types and sources of currently available data, (2) what work is currently in progress, and (3) what are the current and anticipated data needs. We present a GitHub platform for Ariel-related data, with the goal to provide a go-to place for both data-users and data-providers, for the users to make requests for their data needs and for the data-providers to link to their available data. Our aim throughout the paper is to provide practical information on existing sources of data whether in data bases, theoretical, or literature sources.

On the nature of sub-THz continuum absorption in CO2 gas, its mixture with Ar, and in pure water vapor.

Detailed analysis of the unique broadband millimeter-wave (70-360 GHz) collision-induced absorption spectra in pure CO2 and in its mixture with Ar is presented. The nature of the observed continuum absorption is examined using classical trajectory simulation along with statistical physics consideration. Bimolecular continuum is decomposed in the phase space into separate contributions from the so-called free, quasibound, and true bound molecular pairs, the proportions of which greatly vary with temperature. This partitioning is supported by consideration of the second virial coefficient and excluded volume in pure CO2, Ar, and CO2-Ar. Close similarity between collision-induced absorption in the CO2 containing gases and the water vapor continuum in the subterahertz spectral range is demonstrated. This similarity suggests that the physical principles underlying both continuum absorption phenomena have much in common and, therefore, can be used for continuum modeling.

Earth as a Transiting Exoplanet: A Validation of Transmission Spectroscopy and Atmospheric Retrieval Methodologies for Terrestrial Exoplanets

The James Webb Space Telescope (JWST) will enable the search for and characterization of terrestrial exoplanet atmospheres in the habitable zone via transmission spectroscopy. However, relatively little work has been done to use solar system data, where ground truth is known, to validate spectroscopic retrieval codes intended for exoplanet studies, particularly in the limit of high resolution and high signal-to-noise ratio (S/N). In this work, we perform such a validation by analyzing a high-S/N empirical transmission spectrum of Earth using a new terrestrial exoplanet atmospheric retrieval model with heritage in solar system remote sensing and gaseous exoplanet retrievals. We fit the Earth’s 2–14 μm transmission spectrum in low resolution (R = 250 at 5 μm) and high resolution (R = 100,000 at 5 μm) under a variety of assumptions about the 1D vertical atmospheric structure. In the limit of noiseless transmission spectra, we find excellent agreement between model and data (deviations <10%) that enable the robust detection of H2O, CO2, O3, CH4, N2, N2O, NO2, HNO3, CFC-11, and CFC-12 thereby providing compelling support for the detection of habitability, biosignature, and technosignature gases in the atmosphere of the planet using an exoplanet-analog transmission spectrum. Our retrievals at high spectral resolution show a marked sensitivity to the thermal structure of the atmosphere, trace gas abundances, density-dependent effects, such as collision-induced absorption and refraction, and even hint at 3D spatial effects. However, we used synthetic observations of TRAPPIST-1e to verify that the use of simple 1D vertically homogeneous atmospheric models will likely suffice for JWST observations of terrestrial exoplanets transiting M dwarfs.

“The spatial distributions of degraded craters and valley networks on Mars as a function of elevation: Their implications for Noachian climate”

Cold and dry models of Noachian Mars postulate that the Noachian epoch characterized a hyperarid and frigid world dominated by low-latitude cold-locked icefields. This ice caps supposedly protected underlying substrates and topography of the Southern Highlands from erosional degradation. I test the Late Noachian Icy Highlands (LNIH) hypothesis by mapping small distribution of both degraded and fresh [2–4] km diameter craters, crater depth and infilling of the smallest [2–5] km craters, valley network (VN) and inverted channels within an area of interest (AOI) representing over 1.1 × 107 km2 of the Noachian Southern Highlands utilizing Environmental Systems Research Institute (ESRI) ArcMap, clipped exclusively to Noachian terrain. Crater degradation results from this study do not show preferential preservation of fresh craters at elevation. Also, crater rim – crater floor (km) depth values show greatest erosion at elevations >1 km purported LNIH Equilibrium Line Altitude (ELA). VN and inverted channels likewise give evidence for net precipitation and erosion at elevation and net deposition and water transport below the global dichotomy at 0 km and extinction at −2.78 km. VN distribution patterns also requires an intertropical convergence zone (ITCZ). I propose that my results are most consistent with emerging PMGCMs incorporating atmospheric collision-induced-absorption (CIA) of CO2 and H2, bypassing the Faint Young Sun Paradox. I propose that statistical evidence derived from geomorphological features on Mars should be used as boundary conditions for future Mars global climate models.

Parameterized model to approximate theoretical collision-induced absorption band shapes for O2-O2 and O2-N2

Collision-induced absorption from vibronic transitions of O2-O2 and O2-N2 collision complexes is an important contributor to light-matter interaction in the atmosphere with relevance to radiative heat transfer and remote sensing. Despite in-depth studies involving quantum calculations of this effect, comparisons between experiment and theory would benefit from less computationally burdensome calculations that closely approximate the temperature dependence of the theoretical band shape and intensity. Accordingly, we present a parameterized representation of recent quantum calculations for collision-induced absorption by O2-O2 and O2-N2 about the 1.27 μm monomer band of O2. This approach leverages the theoretical decomposition of the spectra into exchange and spin-orbit contributions, with each component having a distinct temperature-dependent band shape. Composite spectra are approximated as a linear combination of the theoretical profiles with seven adjustable parameters that scale the component intensities and their dependences on temperature. We demonstrate that this empirical representation can accurately simulate the theoretical calculations, and we present a global fit of the model to previously reported collision-induced absorption spectra (O2-O2, O2-N2, and O2-air) measured by cavity ring-down spectroscopy over the temperature range 271 K–332 K.

Collision-induced absorption spectra (CIA) for monatomic gas mixtures of Helium-Xenon

The lineshapes of collision-induced absorption (CIA) at room temperature are computed quantum mechanically for gaseous binary mixtures of helium with xenon using theoretical induced dipole moment and interatomic potentials. Empirical and literature models of the induced dipole moments which reproduce the experimental spectra and the first few spectral moments are given. Good agreement between the computed and experimental lineshapes of absorption is obtained when the potential parameters which are fitted well to the vibrational energy levels, thermophysical and transport properties are used.

Accurate absolute frequency measurement of the S(2) transition in the fundamental band of H2 near 2.03 μm.

A series of spectra of the quadrupolar electric S(2) transition of H2 in the 1-0 band near 4917 cm-1 has been recorded at seven pressure values between 2 and 100 Torr. The comb-referenced cavity ring down spectroscopy (CR-CRDS) technique was used for the recording of this very weak transition. The accuracy of the spectrum frequency axis is achieved by linking the CRDS setup to an optical frequency comb referenced to a GPS-referenced 10 MHz rubidium clock. Applying a multi-spectrum fit procedure to the seven averaged spectra with a quadratic speed dependence Nelkin-Ghatak profile, the transition frequency is determined (ν0 = 147 408 142 357 kHz) with an uncertainty of 150 kHz (∼1 × 10-9 in relative). This represents the smallest uncertainty achieved so far for a transition in the fundamental band of H2. The experimental frequency reported in this work is 1.53 MHz higher than the best-to-date theoretical value. This difference represents 1.5 times the 1σ-uncertainty (about 1 MHz) of the calculated frequency. The measurements also allow for the determination of the absolute intensity value of the S(2) line which shows an agreement with the ab initio value at the per mil level. In addition, the cross section of the collision induced absorption (CIA) underlying the S(2) line is accurately retrieved from the quadratic pressure dependence of the baseline level of the recorded spectra.

Accurately computed H2–He collision-induced absorption coefficients for modeling of planetary atmospheres

Accurate collision-induced absorption profiles for H2–He pairs, in the rototranslational band of H2, are computed accounting for the full anisotropic interaction potential. The calculations are time consuming and complicated compared to those pursued in the isotropic potential approximation. A machine learning approach is implemented in order to produce highly accurate data on a dense frequency grid, by combining data computed in the full calculation with those from the isotropic approximation. Thus an extensive, highly accurate, data base can be obtained for a set of frequencies, temperatures, and ortho-H2/para-H2 fractions, appropriate for use in modeling of planetary atmospheres, in particular of the gas giants.

Updated Low-temperature Gas Opacities with ÆSOPUS 2.0

This work introduces new low-temperature gas opacities in the range computed with the ÆSOPUS code under the assumption of thermodynamic equilibrium. In comparison to the previous version, ÆSOPUS 1.0, we updated and expanded molecular absorption to include 80 species, mostly using the recommended line lists currently available from the ExoMol and HITRAN databases. Furthermore, in light of a recent study, we revised the H− photodetachment cross section, added the free–free absorption of other negative ions of atoms and molecules, and updated the collision-induced absorption due to H2/H2, H2/H, H2/He, and H/He pairs. Using the new input physics, we computed tables of Rosseland mean opacities for several scaled-solar chemical compositions, including Magg et al.'s most recent one, as well as α-enhanced mixtures. The differences in opacity between the new ÆSOPUS 2.0 and the original ÆSOPUS 1.0 versions, as well as other sets of calculations, are discussed. The new opacities are released to the community via a dedicated webpage that includes both precomputed tables for widely used chemical compositions and a web interface for calculating opacities on the fly for any abundance distribution.

Spectral analysis of ultra-cool white dwarfs polluted by planetary debris

ABSTRACT We identify two ultra-cool (${T_\mathrm{eff}}\lt 4000$ K) metal-polluted (DZ) white dwarfs WD J2147−4035 and WD J1922+0233 as the coolest and second coolest DZ stars known to date with ${T_\mathrm{eff}}\approx 3050$ K and ${T_\mathrm{eff}}\approx 3340$ K, respectively. Strong atmospheric collision-induced absorption (CIA) causes the suppression of red optical and infrared flux in WD J1922+0233, resulting in an unusually blue colour given its low temperature. WD J2147−4035 has moderate infrared CIA yet has the reddest optical colours known for a DZ white dwarf. Microphysics improvements to the non-ideal effects and CIA opacities in our model atmosphere code yields reasonable solutions to observations of these ultra-cool stars. WD J2147−4035 has a cooling age of over 10 Gyr which is the largest known for a DZ white dwarf, whereas WD J1922+0233 is slightly younger with a cooling age of 9 Gyr. Galactic kinematics calculations from precise Gaia EDR3 astrometry reveal these ultra-cool DZ stars as likely members of the Galactic disc thus they could be pivotal objects in future studies constraining an upper age limit for the disc of the Milky Way. We present intermediate-resolution spectroscopy for both objects, which provides the first spectroscopic observations of WD J2147−4035. Detections of sodium and potassium are made in both white dwarfs, in addition to calcium in WD J1922+0233 and lithium in WD J2147−4035. We identify the magnetic nature of WD J2147−4035 from Zeeman splitting in the lithium line and also make a tentative detection of carbon, so we classify this star as DZQH. WD J1922+0233 likely accreted planetary crust debris, while the debris composition that polluted WD J2147−4035 remains unconstrained.

Collision-induced absorption in Ar-Xe: a comparative study of empirical and ab initio interaction potentials and electric dipole moments

Empirical Barker-Fisher-Watts and modified Tang-Toennies potential energy curves are obtained through fit to experimental vibrational transition energies for argon–argon, xenon–xenon, and argon–xenon pairs. The potentials are tested against experimental thermophysical and transport properties, and agreement is observed. Also, an interaction-induced electric dipole moment curve for the argon–xenon pair is determined through a fit to experimental spectral moments for collision-induced absorption. The argon–xenon potentials and dipole are tested in a complete quantum dynamical calculation of the collision-induced absorption profiles, which can be compared with a laboratory measurement. This provides further analysis of the accuracy of the empirical argon–xenon data, as calculations of absorption profiles are highly sensitive to the input of molecular data.

Extremely high spectral resolution measurements of the 450 µm atmospheric window at Chajnantor with APEX,

Ground-based telescopes observing at millimeter (mm) and submillimeter (submm) wavelengths have to deal with a line-rich and highly variable atmospheric spectrum, both in space and time. Models of this spectrum play an important role in planning observations that are appropriate for the weather conditions and also calibrating those observations. Through magnetic dipolar (M1) rotational transitions and electric dipolar (E1) transitions O2 and H2O, respectively, dominate the atmospheric opacity in this part of the electromagnetic spectrum. Although O2 lines, and more generally the so-called dry opacity, are relatively constant, the absorption related to H2O can change by several orders of magnitude leading from a totally opaque atmosphere near sea level with high H2O columns to frequency windows with good transmission from high and dry mountain sites. Other minor atmospheric gases, such as O3 and N2O among others, are present in the atmospheric spectrum which also includes nonresonant collision-induced absorption due to several mechanisms. The aim of our research is to improve the characterization of the mm/submm atmospheric spectrum using very stable heterodyne receivers with excellent sideband separation and extremely high (kHz) spectral resolutions at the 5000 m altitude Chajnantor site in northern Chile. This last aspect (spectral resolution) is the main improvement (by more than three orders of magnitude) in the presented data with respect to our previous work conducted ~20 yr ago from Mauna Kea in Hawai’i. These new measurements have enabled us to identify slight modifications needed in the Atmospheric Transmission at Microwaves (ATM) model to better take into account minor constituent vertical profiles, include a few missing lines, and adjust some high-energy O3 line frequencies. After these updates, the ATM model is highly consistent with all data sets presented in this work (within ~2% at 1 GHz resolution).

Numerical Calculation of Three-Dimensional Ground State Potential Energy Function of Na2F System

We present a new three-dimensional global potential energy surface (PES) for the ground state of Na2F system. A total of about 1460 points were generated for the PES. All of the points have been carried out by using the coupled-cluster single-, double-, and perturbative triple-excitations [CCSD(T)]. Two Jacobi coordinates, R and θ , and the frozen molecular equilibrium geometries were used. We mixed the basis sets of aug-cc-pCVQZ for the sodium atom and the basis sets of aug-cc-pCVDZ for the fluorine atom with an additional (3s3p2d) set of midbond functions; the energies obtained were extrapolated to the complete basis set limit. The whole calculation adopted supramolecular approximation approach. We divided the potential energy surface into three regions, the peak region, the well region, and the long range region, and calculate the single point energy, respectively. Our ab initio calculations will be useful for future studies of the collision-induced absorption for the Na2-F dimer, and it can be used for modeling the dynamical behavior in Na2F system too.

Potential long-term habitable conditions on planets with primordial H–He atmospheres

Cold super-Earths that retain their primordial, H–He-dominated atmosphere could have surfaces that are warm enough to host liquid water. This would be due to the collision-induced absorption of infrared light by hydrogen, which increases with pressure. However, the long-term potential for habitability of such planets has not been explored yet. Here we investigate the duration of this potential exotic habitability by simulating planets of different core masses, envelope masses and semi-major axes. We find that terrestrial and super-Earth planets with masses of ~1–10 M⊕ can maintain temperate surface conditions up to 5–8 Gyr at radial distances larger than ~2 au. The required envelope masses are ~10−4 M⊕ (which is 2 orders of magnitude more massive than Earth’s) but can be an order of magnitude smaller (when close-in) or larger (when far out). This result suggests that the concept of planetary habitability should be revisited and made more inclusive with respect to the classical definition.