Yves Benilan

Photodestruction of Relevant Interstellar Molecules in Ice Mixtures

UV photodestruction of some interstellar molecules is studied in different kinds of ices. CH4 ,C H 3OH, NH3 ,C O2, CO, and HNCO are photolyzed as pure ices, or mixed with water or molecular nitrogen, at about 10 K. The destruction cross sections of these molecules are estimated for use in photochemical models of interstellar ices. We show that the destruction rate depends on the ice in which the studied compound is embedded. Subject Headings: astrochemistry — ISM: molecules — methods: laboratory — molecular processes — ultraviolet: ISM

Polyoxymethylene as Parent Molecule for the Formaldehyde Extended Source in Comet Halley

Among unsolved questions raised by observations of comets is the origin of extended sources, i.e., the distribution of molecules in the coma which cannot be explained by a direct sublimation from the nucleus. Polyoxymethylene [formaldehyde polymer: also called POM] is sometimes ([CH 2 -O[) n , invoked as a potential parent compound, the degradation of which could produce the required amount of across the coma, but no quantitative study has ever been undertaken with relevant parameters. H 2 CO From new experimental data, we are now able to consider multiphase chemistry: POM in the solid state on cometary grains slowly degrades by solar photons and heat and produces in the gaseous H 2 CO phase. This is a new approach to cometary organic chemistry. We show, by considering simple assumptions about the cometary environment, that the hypothesis of POM on grains leads to a very good agreement with Giotto observations if we assume that the cometary grains are D7% POM by mass at a temperature of 330 K.

High-temperature measurements of VUV-absorption cross sections of CO2 and their application to exoplanets

Context. Ultraviolet (UV) absorption cross sections are an essential ingredient of photochemical atmosphere models. Exoplanet searches have unveiled a large population of short-period objects with hot atmospheres, very different from what we find in our solar system. Transiting exoplanets whose atmospheres can now be studied by transit spectroscopy receive extremely strong UV fluxes and have typical temperatures ranging from 400 to 2500 K. At these temperatures, UV photolysis cross section data are severely lacking. Aims. Our goal is to provide high-temperature absorption cross sections and their temperature dependency for important atmospheric compounds. This study is dedicated to CO2, which is observed and photodissociated in exoplanet atmospheres. We also investigate the influence of these new data on the photochemistry of some exoplanets. Methods. We performed these measurements with synchrotron radiation as a tunable VUV light source for the 115–200 nm range at 300, 410, 480, and 550 K. In the 195–230 nm range, we used a deuterium lamp and a 1.5 m Jobin-Yvon spectrometer and we worked at seven temperatures between 465 and 800 K. We implemented the measured cross section into a 1D photochemical model. Results. For λ> 170 nm, the wavelength dependence of ln(σCO2 (λ,T ) × 1 Qv(T ) ) can be parametrized with a linear law. Thus, we can interpolate σCO 2 (λ,T ) at any temperature between 300 and 800 K. Within the studied range of temperature, the CO2 cross section can vary by more than two orders of magnitude. This, in particular, makes the absorption of CO2 significant up to wavelengths as high as 230 nm, while it is negligible above 200 nm at 300 K. Conclusions. The absorption cross section of CO2 is very sensitive to temperature, especially above 160 nm. The model predicts that accounting for this temperature dependency of CO 2 cross section can affect the computed abundances of NH 3 ,C O 2 , and CO by one order of magnitude in the atmospheres of hot Jupiter and hot Neptune. This effect will be more important in hot CO 2 -dominated atmospheres.

THE TEMPERATURE DEPENDENT ABSORPTION CROSS SECTIONS OF C4H2 AT MID ULTRAVIOLET WAVELENGTHS

Abstract We have measured the mid UV (195–265 nm) absorption coefficients of diacetylene, C4H2, over the temperature range 193–293 K. An impurity involved in the synthesis of C4H2, namely C4H3Cl, can strongly influence the measured absorption coefficients. This impurity has a mid UV absorption approximately 300 times stronger than C4H2 and we demonstrate that its presence, at low levels, in previously published spectra has strongly influenced the published coefficients and their temperature dependence. We have obtained an ultra-pure sample of C4H2 by repeated distillation and show here its clean spectrum and temperature dependence. We confirm previous band assignments, but our superior resolution (0.02 nm) has shown the presence of other, previously undetected bands. These new results have wide implications for the modelling of Titans atmosphere and the analysis of observational data.

Characterization of an N2 flowing microwave post-discharge by OES spectroscopy and determination of absolute ground-state nitrogen atom densities by TALIF

A flowing microwave post-discharge source sustained at 2.45 GHz in pure nitrogen has been investigated by optical emission spectroscopy (OES) and two-photon absorption laser-induced fluorescence (TALIF) spectroscopy. Variations of the optical emission along the post-discharge (near, pink and late afterglow) have been studied and the gas temperature has been determined. TALIF spectroscopy has been used in the late afterglow to determine the absolute ground-state nitrogen atomic densities using krypton as a reference gas. Measurements show that the microwave flowing post-discharge is an efficient source of N ( 4 S) atoms in late afterglow. In our experimental conditions, the maximum N ( 4 S) density is about 2.2 × 10 15 cm −3 for a pressure of 22 Torr, at 300 K. The decay of N ( 4 S) density as a function of the time spent in the quartz tube has been modelled and a wall recombination probability γ of (2.1 ± 0.3) × 10 −4 is obtained. (Some figures in this article are in colour only in the electronic version)

The long wavelength range temperature variations of the mid-UV acetylene absorption coefficient

In the reductive atmospheres of the giant planets and Titan, acetylene is known to be the major unsaturated hydrocarbon. It is of great importance to determine precisely and to model its abundance profile in order to be able to fully understand the chemistry of these environments. To achieve this task one needs the knowledge of the absorption coeAcient in IR and UV which are complementary wavelength ranges for studying atmospheres. The mid-UV absorption coeAcient is of special importance when trying to model the photo-dissociation of C2H2 because of the rise of the solar flux above 200 nm. We have previously shown that the most recent data on acetylene cross sections had to be taken with caution because of the presence of acetone bands in the published spectra. Moreover, absolute absorption coeAcient of C2H2 is poorly known above 200 nm. Consequently, we have measured C2H2 absorption coeAcient in the 185‐235 nm range, at 295 and 173 K. We present the obtained results, putting a special emphasis on their temperature dependence. Then, we discuss the implications of those results on theoretical photochemistry modeling and on future observations of methane rich atmospheres. 7 2000 Elsevier Science Ltd. All rights reserved.

Ultraviolet and infrared spectrum of C6H2 revisited and vapor pressure curve in Titan's atmosphere

Abstract The presence of the linear molecules called polyynes, (C2nH2, n>2), in Titans atmosphere is suggested by the signatures of acetylene C2H2 and of diacetylene C4H2 in Voyager spectra. Both atmospheric simulations and photochemical modelling support polyynes implication in Titans chemistry as an interface between the gaseous phase and the solid phase visible in aerosols form. However, the detection of polyynes higher than C4H2 depends on our ability to determine their spectra in the laboratory under low temperature and pressure conditions. We revisit here spectroscopic investigations on triacetylene, C6H2, since previous UV and IR measurements suffered from great uncertainty, respectively, due to an impurity contribution and saturation effects. We point out the importance of studying a pure sample and we underline the strong temperature dependency of UV absolute absorption coefficients (185– 320 nm ). In the IR range (220– 4300 cm −1 ), our determination of the absolute intensity of the main bands is 30% higher than previous measurements. For the first time, the vapor pressure law of triacetylene is investigated in a limited temperature range (170– 200 K ) allowing a calculation of its enthalpy of sublimation. Those results applied to Titans atmospheric conditions show the possible existence of two condensation regions: one located in the low stratosphere (∼100 km ) and the other in thermosphere (∼700 km ) . The condensation at an altitude of 700 km is consistent with the observation of an upper haze layer. This could imply the presence of a heterogeneous chemistry but also an inhibition of the polyynes formation, not included in available photochemical models.

A New Infrared Band in Interstellar and Circumstellar Clouds: C4 or C4H?

We report on the detection with the Infrared Space Observatory of a molecular band at 57.5 μm (174 cm-1) in carbon-rich evolved stars and in Sgr B2. Taking into account the chemistry of these objects, the most likelihood carrier is a carbon chain. We tentatively assign the band to the ν5 bending mode of C4 for which a wavenumber of 170-172.4 cm-1 has been derived in matrix experiments by Withey et al. An alternate carrier might be C4H, although the frequency of its lowest energy vibrational bending mode, ν7, is poorly known (130-226 cm-1). If the carrier is C4, the derived maximum abundance is nearly similar to that found for C3 in the interstellar and circumstellar media by Cernicharo et al. Hence, tetra-atomic carbon could be one of the most abundant carbon chain molecules in these media.

Mid‐UV spectroscopy of propynenitrile at low temperature: Consequences on expected results from observations of Titan's atmosphere

The interpretation of albedo spectra in the mid-UV range is critically dependent on the knowledge of the absolute absorption coefficient, at specific temperatures and with high resolution for molecular compounds likely to absorb the scattered solar radiation in this wavelength range. Propynenitrile, a key molecule in prebiotic synthesis, already detected with IR and microwave sounding in Titans atmosphere, exhibits intense absorption bands in the 185 to 230 nm interval. This paper deals with the absolute absorption coefficient k of propynenitrile at low temperature and high resolution and compares it to the room temperature values. The consequences of the observed k variations on the interpretation of Titans atmosphere spectra, expected in the near future, are considered here. As an additional result, the pressure regulation system of the experiment allows us to assess the determination of the temperature dependence of the propynenitrile vapor pressure.

The nu(8) bending mode of diacetylene: from laboratory spectroscopy to the detection of (13)C isotopologs in Titan's atmosphere

The strong nu(8) band of diacetylene at 627.9 cm(-1) has been investigated to improve the spectroscopic line data used to model the observations, particularly in Titans atmosphere by Cassini/Composite Infrared Spectrometer. Spectra have first been recorded in the laboratory at 0.5 and 0.1 cm(-1) resolution and temperature as low as 193 K. Previous analysis and line lists present in the GEISA database appeared to be insufficient to model the measured spectra in terms of intensity and hot band features. To improve the situation and in order to be able to take into account all rovibrational transitions with a non-negligible intensity, a global analysis of C4H2 has been carried out to improve the description of the energy levels up to E-v = 1900 cm(-1). The result is a new extensive line list which enables us to model very precisely the temperature variation as well as the numerous hot band features observed in the laboratory spectra. One additional feature, observed at 622.3 cm(-1), was assigned to the nu(6) mode of a C-13 isotopologue of diacetylene. The nu(8) bands of both C-13 isotopomers were also identified in the 0.1 cm(-1) resolution spectrum. Finally, a C-13/C4H2 line list was added to the model for comparison with the observed spectra of Titan. We obtain a clear detection of C-13 monosubstituted diacetylene at 622.3 cm(-1) and 627.5 cm(-1) (blended nu(8) bands), deriving a mean C-12/C-13 isotopic ratio of 90 +/- 8. This value agrees with the terrestrial (89.4, inorganic standard) and giant planet values (88 +/- 7), but is only marginally consistent with the bulk carbon value in Titans atmosphere, measured in CH4 by Huygens GCMS to be 82 +/- 1, indicating that isotopic fractionation during chemical processing may be occurring, as suggested for ethane formation.