Characterization of aromaticity in analogues of titan's atmospheric aerosols with two-step laser desorption ionization mass spectrometry

The role of polycyclic aromatic hydrocarbons (PAH) and Nitrogen containing PAH (PANH) as intermediates of aerosol production in the atmosphere of Titan has been a subject of controversy for a long time. An analysis of the atmospheric emission band observed by the Visible and Infrared Mapping Spectrometer (VIMS) at 3.28 micrometer suggests the presence of neutral polycyclic aromatic species in the upper atmosphere of Titan. These molecules are seen as the counter part of negative and positive aromatics ions suspected by the Plasma Spectrometer onboard the Cassini spacecraft, but the low resolution of the instrument hinders any molecular speciation. In this work we investigate the specific aromatic content of Titan's atmospheric aerosols through laboratory simulations. We report here the selective detection of aromatic compounds in tholins, Titan's aerosol analogues, produced with a capacitively coupled plasma in a N2:CH4 95:5 gas mixture. For this purpose, Two-Step Laser Desorption Ionization Time-of-Flight Mass Spectrometry (L2DI-TOF-MS) technique is used to analyze the so produced analogues. This analytical technique is based on the ionization of molecules by Resonance Enhanced Multi-Photon Ionization (REMPI) using a {\lambda}=248 nm wavelength laser which is selective for aromatic species. This allows for the selective identification of compounds having at least one aromatic ring. Our experiments show that tholins contain a trace amount of small PAHs with one to three aromatic rings. Nitrogen containing PAHs (PANHs) are also detected as constituents of tholins. Molecules relevant to astrobiology are detected as is the case of the substituted DNA base adenine

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

VUV photophysics of acetic acid: Fragmentation, fluorescence and ionization in the 6 23 eV region

International audienceVUV photodissociation of gaseous acetic acid was studied in the 6-23 eV range using synchrotron radiation excitation, photofragment fluorescence spectroscopy and mass spectrometry. OH (A-X), CH (A,B-X) and H-Balmer emissions were observed. Their relative intensities were studied by fluorescence excitation spectroscopy. The fluorescence quantum yield for OH emission has a maximum of 0.9% at 13.3 eV photoexcitation, dropping to 0.5% at 20 eV; that for CH (A-X) is 0.35% at 16 eV and 0.4% at 20 eV. Photoionization mass spectra (PIMS) of CH 3COOH were measured and the appearance energies of the principal photoions were determined. IE(CH 3COOH) = 10.58 ± 0.02 eV is 40-60 meV lower than previous PIMS values. Dissociative ionization reaction channels are discussed in detail. The results call into question previous determinations of the heat of formation and ionization energy of the acetyl radical. A new pathway is suggested for the formation of HCO +, and the assignments of the m/ z = 16, 28 and 31 ions are clarified. The formation of CH3+ at threshold is shown to involve carbon-carbon bond rupture and a potential energy barrier. The results of this study are used to discuss aspects of astrophysical observations involving the parent and fragment species

The two-top molecule 3-penten-2-one: Acetyl methyl torsion in α,β-unsaturated ketones

International audienceUsing a molecular jet Fourier transform spectrometer, the microwave spectrum of E-3-penten-2-one, also called methyl propenyl ketone, was recorded in a range from 2.0 to 26.5 GHz. Two conformers, antiperiplanar (ap) and synperiplanar (sp), could be identified. Complicated splitting patterns arising due to the internal rotation of the acetyl methyl and propenyl methyl groups could be resolved, and all measured rotational transitions were fitted to measurement accuracy. The barrier heights of the acetyl methyl group are 434.149(37) cm−1 and 358.076(26) cm−1 for the ap and sp conformers, respectively. For the propenyl methyl group, the barrier is 581.903(45) cm−1 for the ap conformer and 595.271(71) cm−1 for the sp conformer. By comparing these results to those of other ketones categorized in a class system connecting the internal rotation of the acetyl methyl group to the structures of the molecules, we extended the categorization with a new class for α,β-unsaturated ketones

Photoion mass spectrometry of adenine, thymine and uracil in the 6 22 eV photon energy range

International audienceUsing synchrotron radiation as excitation source in the 6 22 eV photon energy region, a photoionization mass spectrometry study of three nucleic acid bases, adenine, thymine and uracil, revealed VUV-induced degradation pathways of these important biological molecules. The fragmentation patterns, ionization energies and ion appearance energies (AE) are reported, many for the first time, and are compared with results of electron impact and other studies. AE values enabled heats of formation of parent and some fragment ions to be revised or determined for the first time. Thermochemical data, coupled with the observed AEs, were also useful in clarifying dissociative photoionization pathways. The main neutral loss species are HCN for adenine, HNCO and CO for thymine and uracil, but many subsequent and other fragmentation pathways, including some not suggested previously, are observed and discussed. The hyperconjugation properties of the methyl group make CO loss easier in thymine than in uracil. The astrophysically important fragment ion HCNH+ is shown to be formed by several fragmentation pathways in all three nucleobases. The relative importance of competitive fragmentation processes was determined in some cases. Some astrophysical implications concerning the prospects for observation and survival of these nucleic acid bases in the interstellar medium and in meteorites are briefly discussed

VUV photophysics of acetonitrile: Fragmentation, fluorescence and ionization in the 7 22 eV region

International audienceVUV induced photodissociation of gaseous acetonitrile was studied in the 7-22 eV range using synchrotron radiation excitation, photofragment fluorescence spectroscopy and photoion mass spectrometry (PIMS). CN(A-X), CN(B-X) and CH(A-X) emissions were observed. Their relative intensities were studied by fluorescence excitation (FEX) spectroscopy. The fluorescence quantum yield for CN(B-X) emission is of the order of 1.5%, whereas that of CN(A-X) is of the order of 0.5%, relatively independent of excitation wavelength in the 9-20 eV region. The CN FEX spectra follow quite closely the photoabsorption spectrum. The total photoionization quantum yield of CH 3CN was measured up to 22 eV. The structures in the yield curve were related to photoabsorption features. The total photoionization quantum yield is about 0.5 at 13.131 eV, the energy of the first excited state of the ion; it reaches unity at about 21.3 eV. Dissociative reaction channels forming neutral products are discussed in detail. The results of this study and of an associated VUV absorption spectroscopy study of acetonitrile are used to discuss aspects of the interpretations of astrophysical observations of acetonitrile in the interstellar medium, comets and planetary atmospheres

VUV photophysics and dissociative photoionization of pyrimidine, purine, imidazole and benzimidazole in the 7 18 eV photon energy range

International audiencePhotoionization mass spectrometry is used to study ionization processes and fragmentation pathways of four prebiotic species, pyrimidine, purine, imidazole and benzimidazole, in the 7-18 eV photon energy region, with synchrotron radiation as excitation source. These molecules are possible precursors of the nucleic acid bases that occur in DNA and RNA. Ionization energies and ion appearance energies are reported. They are compared with electron impact and other studies and are discussed in terms of the electronic and nuclear structures of these species and their cations. The ion appearance energies, in conjunction with thermochemical data, were used to propose dissociative photoionization pathways, principally involving loss of HCN molecules in each of the four species. Astrophysical implications of the results concern the prospects for observation and survival of these molecules in the interstellar medium, in comets and in meteorites. Suggestions are made concerning suitable sites for radioastronomical searches for these purines and pyrimidines

Gas-phase conformations of 2-methyl-1,3-dithiolane investigated by microwave spectroscopy

International audienceThe conformational analysis of 2-methyl-1,3-dithiolane using quantum chemical calculations at some levels of theory yielded only one stable conformer with envelope geometry. However, other levels of theory indicated two envelope conformers. Analysis of the microwave spectrum recorded using two molecular jet Fourier transform microwave spectrometers covering the frequency range from 2 to 40.0 GHz confirms that only one conformer exists under jet conditions. The experimental spectrum was reproduced using a rigid-rotor model with centrifugal distortion correction within the measurement accuracy of 1.5 kHz, and molecular parameters were determined with very high accuracy. The gas phase structure of the title molecule is compared with the structures of other related molecules studied under the same experimental conditions

VUV absorption spectrum of acetic acid between 6 and 20 eV

International audienceAbsorption spectra of acetic acid were measured between 6 and 20 eV at a resolution of 8 meV. Previous measurements had a spectral limit of 11.7 eV. Analysis and band assignment were aided by data from theoretical calculations on valence states and from photoelectron spectroscopy. Valence transitions and nsa