Antoine Jolly

Capacitively coupled plasma used to simulate Titan's atmospheric chemistry

A complex chemistry in Titan’s atmosphere leads to the formation of organic solid aerosols. We use a radio-frequency (RF) capacitively coupled plasma discharge produced in different N2–CH4 mixtures (from 0% to 10% of CH4) to simulate this chemistry. The work presented here was devoted to the study of the plasma discharge. In our experiment, the electron density is measured by the resonant cavity method and is about 10 15 m −3 in pure N2 plasma at 30 W excitation RF power. It decreases by a factor of 2 as soon as CH4 is present in the discharge, even for a proportion as small as 2% of CH4. An optical emission spectroscopy diagnostic is installed on the experiment to study the evolution of the N2 bands and to perform actinometry measurements using Ar lines. This diagnostic allowed us to measure variations in the electron temperature and to show that a decrease in the density of the electrons can be compensated by an increase in their energy. We have also used an experimental setup where the plasma is tuned in a pulsed mode, in order to study the formation of dust particles. We observed variations in the self-bias voltage, the RF injected power and the intensities of the nitrogen bands, which indicated that dust particles were formed. The characteristic dust formation time varied, depending on the experimental conditions, from 4 to 110 s. It was faster for higher pressures and for smaller proportions of CH4 in the gas mixture. (Some figures in this article are in colour only in the electronic version)

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.

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.

ISOTOPIC RATIOS IN TITAN'S ATMOSPHERE FROM CASSINI CIRS LIMB SOUNDING : HC3N IN THE NORTH

Isotopic ratios in planetary atmospheres are valuable sources of information regarding the formation and evolution of the body. Their present-day values may also be ambiguous, reflecting the sum total of many processes that can alter the ratio from primordial: physical and chemical processes in the nebula, during primary or secondary atmospheric formation, and subsequently. For this reason, deducing a unique past history for a given isotopic ratio is rarely straightforward.

Optimization of a Solar Simulator for Planetary-photochemical Studies

Low-temperature microwave-powered plasma based on hydrogen and hydrogen with noble gas mixtures are widely used as a continuous vacuum ultraviolet (VUV) source in laboratory experiments carried out to mimic the photochemistry in astrophysical environments. In this work, we present a study dedicated to optimizing such sources in terms of mono-chromaticity at Lyα (H(Lyα) line at 121.6 nm ~ 10.2 eV) and high spectral irradiance. We report the influence on the emission spectrum of a wide range of experimental conditions including gas composition (pure H2, pure He, and H2/He mixture), gas pressure, flow rates, and microwave power. The absolute spectral irradiance delivered by this VUV light source has been measured. With a microwave input power of 100 W, the best conditions for producing a quasi-monochromatic source are a 1% H2/He gas mixture at a total pressure of 5 mbar and a flow rate of 2 sccm. By changing the microwave input power from 30 to 120 W, H(Lyα) increases by more than one order of magnitude. A comparison between the current measurements and the solar VUV spectral irradiance is reported over 115–170 nm.

Absolute ground-state nitrogen atom density in a N2/CH4 late afterglow: TALIF experiments and modelling studies

Following a first study on a late afterglow in flowing pure nitrogen post discharge, we report new two-photon absorption laser-induced fluorescence (TALIF) measurements of the absolute ground-state atomic nitrogen density N(4S) and investigate the influence of methane introduced downstream from the discharge by varying the CH4 mixing ratio from 0% up to 50%. The N (4S) maximum density is about 2.2 × 1015 cm−3 in pure N2 for a residence time of 22 ms and does not change significantly for methane mixing ratio up to ~15%, while above, a drastic decrease is observed. The influence of the residence time has been studied.A kinetic model has been developed to determine the elementary processes responsible for the evolution of the N (4S) density in N2/CH4 late afterglow. This model shows the same decrease as the experimental results even though absolute density values are always larger by about a factor of 3. In the late afterglow three-body recombination dominates the loss of N (4S) atoms whatever the CH4 mixing ratio. For high CH4 mixing ratio, the destruction process through collisions with CH3, H2CN and NH becomes important and is responsible for the observed decrease of the N (4S) density.

Laboratory experiments as support to the built up of Titan’s theoretical models and interpretation of Cassini-Huygens data

To interpret the concentrations of the products measured in Titan’s atmosphere and to better understand the associated chemistry, many theoretical models have been developed so far. Unfortunately, large discrepancies are still found between theoretical and observational data. A critical examination of the chemical scheme included in these models points out some problems regarding the reliability of the description of critical reaction pathways as well as the accuracy of kinetic parameters. Laboratory experiments can be used to reduce these two sources of uncertainty. It can be: i) experimental simulations: in our laboratory (LISA), representative Titan’s simulation experiments are planned to be carried out in a reactor where the initial gas mixture will be exposed, for the first time, to both electrons and photons. Thus, the chemistry between N atoms and CH3 , CH2 , CH fragments, issued from electron dissociation of N2 and photo-dissociation of CH4 respectively, will be initiated. Thank to a time resolved technique, we will be able to analyse “in situ”, qualitatively and quantitatively, the stable species as well as the short life intermediates. Then, the implied chemistry will be determined precisely, and consequently, its description will be refined in theoretical models. The current status of this program will be given. ii) specific experiments: they are devoted, for example, to determine kinetic rate constants and low temperature VUV spectra that will be used to feed models and to interpret observational data. Such experiments performed in LISA and in Rennes’ laboratory concern polyynes and cyanopolyynes as these compounds could link the gaseous and the solid phase in planetary atmosphere. Results concerning C4H hydrocarbons kinetic rate constants and VUV cross section of HC3N and HC5N will be detailed.

The new Titan: an astrobiological perspective

Since the first Voyager data, Titan, the largest satellite of Saturn and only satellite in the solar system having a dense atmosphere, became one of the key planetary bodies for astrobiological studies, due to: i) its many analogies with planet Earth, in spite of much lower temperatures, ii) the already well observed presence of an active organic chemistry, involving several of the key compounds of prebiotic chemistry, in the gas phase but also assumed to occur in the solid phase through the haze particles. And the potential development of a prebiotic chemistry in liquid water, with a possible water ocean in its internal structure, and the possible episodic formation of small liquid water bodies for short but not negligible time duration at the surface (from the melting of surface water ice by impact), iii) the resulting possibility that life may have emerged on or in Titan and may have been able to adapt and to persist. These aspects are examined with some of the associated questions on the basis of the already available Cassini-Huygens data.