New benzene absorption cross sections in the VUV, relevance for Titan’s upper atmosphere

This is a pre-print (pre-peer review) manuscript. It is moderately different from the accepted manuscript and from the published article. Citation of published article: Fernando J. Capalbo, Yves Bénilan, Nicolas Fray, Martin Schwell, Norbert Champion, Et-touhami Es-sebbar, Tommi T. Koskinen, Ivan Lehocki, Roger V. Yelle. Icarus, vol. 265, p. 95 - 109. February 2016. doi: 10.1016/j.icarus.2015.10.006.International audienceBenzene is an important molecule in Titan’s atmosphere because it is a potential link between the gas phase and the organic solid phase. We measured photoabsorption in the ultraviolet by benzene gas at temperatures covering the range from room temperature to 215 K. We derived benzene absorption cross sections and analyzed them in terms of the transitions observed. No significant variation with measurement temperature was observed. We discuss the implications of our measurements for the derivation of benzene abundance profiles in Titan’s thermosphere, by the Cassini/Ultraviolet Imaging Spectrograph (UVIS). The use of absorption cross sections at low temperature is recommended to avoid small systematic uncertainties in the profiles. We used our measurements, together with absorption cross sections from other molecules, to analyze four stellar occultations by Titan, measured by UVIS during flybys T21, T41, T41_II, and T53. We derived and compared benzene abundance profiles in Titan’s thermosphere between approximately 530 and 1000 km, for different dates and geographical locations. The comparisons of our benzene profiles with each other, and with profiles from models of the upper atmosphere, point to a complex behavior that is not explained by current photochemical models

Semi-empirical calculation of electronic absorption wavelengths of polyynes, monocyano- and dicyanopolyynes. Predictions for long chain compounds and carbon allotrope carbyne

International audienceAbsorption wavelengths and oscillators strengths have been calculated for theallowed low-energy electronic transition $^1\Sigma^+ \leftarrow ^1\Sigma^+$of monocyanopolyynes (HC$_y$N, $y = 1-13$), $^1\Sigma_u^+ \leftarrow^1\Sigma_g^+$ of polyynes and dicyanopolyynes (HC$_y$H and NC$_y$N, $y =1-40$). For $y > 18$, the geometries were extrapolated from DFT optimization ofthe shortest members. Extrapolation formulas have been drawn up for longerchains, the asymptotic values of those yield an estimation of the absorptionwavelength (ca. 400 nm) of the hypothetical carbon allotrope carbyne

Theoretical Study of the Structure and Properties of Polyynes and Monocyano- and Dicyanopolyynes: Predictions for Long Chain Compounds

doi: 10.1021/jp013043qInternational audienceGeometrical parameters and harmonic vibration frequencies of polyynes HC2nH (n = 1−8), cyanopolyynes HC2n-1N (n = 1−7), and dicyanopolyynes NC2nN (n = 1−8) have been calculated with various density functionals using the Dunning triple-ζ basis set. For selected data, we propose extrapolation formulas for longer molecules of the series. Bond lengths and electron localization function analysis indicate that a marked bond alternation persists in molecules as long as HC30H. The evolution along the series of some vibration frequencies and their accuracy for the identification of long molecules in extraterrestrial systems are also discussed

The origin of the CN radical in comets: A review from observations and models

International audienceThe origin of CN radicals in comets is not completely understood so far. We present a study of CN and HCN production rates and CN Haser scale lengths showing that: (1) at heliocentric distances larger than 3 AU, CN radicals could be entirely produced by HCN photolysis; (2) closer to the Sun, for a fraction of comets CN production rates are higher than HCN ones whereas (3) in the others, CN distribution cannot be explained by the HCN photolysis although CN and HCN production rates seem to be similar. Thus, when the comets are closer than 3 AU to the Sun, an additional process to the HCN photolysis seems to be required to explain the CN density in some comets. The photolysis of HC 3N or C 2N 2 could explain the CN origin. But the HC 3N production rate is probably too low to reproduce CN density profile, even if uncertainties on its photolysis leave the place for all possible conclusions. The presence of C 2N 2 in comets is a reliable hypothesis to explain the CN origin; thus, its detection is a challenging issue. Since C 2N 2 is very difficult to detect from ground-based observations, only in situ measurements or space observations could determine the contribution of this compound in the CN origin. Another hypothesis is a direct production of CN radicals by the photo- or thermal degradation of complex refractory organic compounds present on cometary grains. This process could explain the spatial profile of CN inside jets and the discrepancy noted in the isotopic ratio 14N/ 15N between CN and HCN. Laboratory studies of the thermal and UV-induced degradation of solid nitrogenated compounds are required to model and validate this hypothesis

TITAN'S UPPER ATMOSPHERE FROM CASSINI/UVIS SOLAR OCCULTATIONS

This is an author-created, un-copyedited version of an article accepted for publication in The Astrophysical Journal. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at 10.1088/0004-637X/814/2/86.This is a pre-print (pre-peer review) manuscript. It is moderately different from the accepted manuscript and from the published article. Citation of published article: Fernando J. Capalbo, Yves Bénilan, Roger V. Yelle, Tommi T. Koskinen. Titan's Upper Atmosphere from Cassini/UVIS Solar Occultations. The Astrophysical Journal, vol. 814, num. 2, p. 86. December 2015. doi: 10.1088/0004-637X/814/2/86.International audienceTitan’s atmosphere is composed mainly of molecular nitrogen, methane being the principal trace gas. From the analysis of 8 solar occultations measured by the Extreme Ultraviolet (EUV) channel of the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini, we derived vertical profiles of N2 in the range 1100 - 1600 km and vertical profiles of CH4 in the range 850 - 1300 km. The correction of instrument effects and observational effects applied to the data are described. We present CH4 mole fractions, and average temperatures for the upper atmosphere obtained from the N2 profiles. The occultations correspond to different times and locations, and an analysis of variability of density and temperature is presented. The temperatures were analyzed as a function of geographical and temporal variables, without finding a clear correlation with any of them; although a trend of decreasing temperature towards the north pole was observed. The globally averaged temperature obtained is (150 ± 1) K. We compared our results from solar occultations with those derived form other UVIS observations, as well as studies performed with other instruments. The observational data we present confirm the atmospheric variability previously observed, add new information to the global picture of Titan’s upper atmosphere composition, variability and dynamics, and provide new constrains to photochemical models

IR and UV spectroscopic data for polyynes: predictions for long carbon chain compounds in Titan's atmosphere

International audienceA better understanding of the complex organic chemistry occurring in the methane rich atmosphere of Titan can be achieved via the comparison of observations with results obtained by theoretical models. Available observations are still few but their analysis requires the knowledge of a large set of data, namely frequencies and absolute band intensities. Cross sections are also needed to develop the chemical schemes of photochemical models, in particular the schemes leading to the formation of haze particles visible on Titan. Unfortunately, some of these parameters are not well known. especially if one takes into account the extreme physical conditions of the studied object. This lack of data is particularly enhanced for polyynes because these compounds are highly unstable at the usual pressure and temperature conditions of a laboratory and therefore are very difficult to study. We have developed UV and IR studies, coupling experimental and theoretical approaches, in order to extrapolate the parameters available for short polyynes to longer carbon chains. In the mid-UV range, when the length of the chain increases, the absorption system of polyynes is shifted to longer wavelength and its oscillator strength increases linearly. In the IR range, with the increase of the number of carbon bonds, the positions of the CCC and CCH bending modes shift to lower energy, the latest converging rapidly to a fixed value of 620.5 cm−1 for an infinite length polyyne. Implications for detection and evolution of polyynes in Titan's atmosphere are emphasised

High temperature VUV cross-section measurements for the study of hot exoplanets' atmospheres: new line list and temperature dependance of A1Π-X1Σ+ CO transition, 3 ≤v' ≤ 10 and v" = 0, 1

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High temperature VUV cross section measurements of acetylene for hot exoplanets study

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