[1] The absorption by oxygen in the region of the O 2 A band near 760 nm has been measured in the laboratory under various conditions of pressure (20-200 atm) and temperature (200-300 K) for both pure O 2 and O 2 -N 2 mixtures. In order to calculate the contribution of the ''allowed'' A band transitions, Lorentzian profiles and a model accounting for line-mixing (LM) effects using the energy corrected sudden (ECS) approximation have been used. The differences between computed spectra and measured values enable extraction of the collision-induced absorption (CIA) contribution. It is shown that neglecting line mixing overestimates absorption in the wings and underestimates absorption at the P and R branch peaks, whereas the CIA extracted by the line-mixing approach shows the ''smooth'' profile expected. Applying this approach to our spectra enables determination of the CIA and allowed contributions for both O 2 -O 2 and O 2 -N 2 collisions versus temperature and pressure. The resulting model and data are then used to build a database and some software suitable for the calculation of oxygen (in air) atmospheric absorption and for easy inclusion in radiative transfer codes (available upon request). These tools are then applied to a theoretical study of the influences of both line-mixing and collision-induced processes on atmospheric photon path escape factors and on cloud-top altitude retrievals. It is shown that LM and CIA make significant contributions and explain a large part of the discrepancies between measured and calculated atmospheric absorption observed recently.

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A quantum approach and classical molecular dynamics simulations (CMDS) are proposed for the modeling of rotational relaxation and of the nonadiabatic alignment of gaseous linear molecules by a nonresonant laser field under dissipative conditions. They are applied to pure CO(2) and compared by looking at state-to-state collisional rates and at the value of induced by a 100 fs laser pulse linearly polarized along z[overhead arrow]. The main results are: (i) When properly requantized, the classical model leads to very satisfactory predictions of the permanent and transient alignments under non-dissipative conditions. (ii) The CMDS calculations of collisional-broadening coefficients and rotational state-to-state rates are in very good agreement with those of a quantum model based on the energy corrected sudden (ECS) approximation. (iii) Both approaches show a strong propensity of collisions, while they change the rotational energy (i.e., J), to conserve the angular momentum orientation (i.e., M/J). (iv) Under dissipative conditions, CMDS and quantum-ECS calculations lead to very consistent decays with time of the "permanent" and transient components of the laser-induced alignment. This result, expected from (i) and (ii), is obtained only if a properly J- and M-dependent ECS model is used. Indeed, rotational state-to-state rates and the decay of the "permanent" alignment demonstrate, for pure CO(2), the limits of a M-independent collisional model proposed previously. Furthermore, computations show that collisions induce a decay of the "permanent" alignment about twice slower than that of the transient revivals amplitudes, a direct consequence of (iii). (v) The analysis of the effects of reorienting and dephasing elastic collisions shows that the latter have a very small influence but that the former play a non-negligible role in the alignment dynamics. (vi) Rotation-translation collisionally induced transfers have also been studied, demonstrating that they only slightly change the alignment dissipation for the considered laser energy conditions.

Reducing atmospheres have recently emerged as a promising scenario to warm the surface of early Mars enough to drive the formation of valley networks and other ancient aqueous features that have been detected so far on the surface of Mars. Here we present a series of experiments and calculations to better constrain CO2+CH4 and CO2+H2 collision-induced absorptions (CIAs) as well as their effect on the prediction of early Mars surface temperature. First, we carried out a new set of experimental measurements (using the AILES line of the SOLEIL synchrotron) of both CO2+CH4 and CO2+H2 CIAs. These measurements confirm the previous results of Turbet et al. 2019, Icarus vol. 321, while significantly reducing the experimental uncertainties. Secondly, we fitted a semiempirical model to these CIAs measurements, allowing us to compute the CO2+CH4 and CO2+H2 CIAs across a broad spectral domain (0-1500cm -1 ) and for a wide range of temperatures (100-600K). Last, we performed 1-D numerical radiative-convective climate calculations (using the LMD Generic Model) to compute the surface temperature expected on the surface of early Mars for several CO2, CH4 and H2 atmospheric contents, taking into account the radiative effect of these revised CIAs. These calculations demonstrate that thick CO2+H2-dominated atmospheres remain a viable solution for warming the surface of Mars above the melting point of water, but not CO2+CH4-dominated atmospheres. Our calculated CO2+CH4 and CO2+H2 CIA spectra and predicted early Mars surface temperatures are provided to the community for future uses.

International audienc

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Measurements of pure CO(2) absorption in the 2.3-μm region are presented. The 3800-4700-cm(-1) range has been investigated at room temperature for pressures in the 10-50-atm range by using long optical paths. Phenomena that contribute to absorption are listed and analyzed, including the contribution of far line wings as well as those of the central region of both allowed and collision-induced absorption bands. The presence of simultaneous transitions is also discussed. Simple and practical approaches are proposed for the modeling of absorption, which include a line-shape correction factor χ that extends to approximately 600 cm(-1) from line centers.

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