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Novel Analysis Toward UndeRstAnding the moLecular complexity in the InterStellar Medium

Final Report Summary - NATURALISM (Novel Analysis Toward UndeRstAnding the moLecular complexity in the InterStellar Medium.)

The final steps of the implementation of the MATRIICES experimental setup has turned out to be much more complicated than expected. The initial sensitivity of the experimental setup has been measured to be about 0.3 percent, which is better that regular analysis techniques (FTIR), but unfortunately not enough to reach our objectives. This has delayed the planned tasks consisting of identifying the largest reaction products (i.e. more than 10 atoms) of O-bearing and N-bearing ices, relevant to the interstellar medium under vacuum ultraviolet (VUV) photolysis. Simpler systems had to be tested first in order to optimise the time-of-flight signal. Although the combination of laser-desorption post-ionization and time-of-flight mass spectrometry is widely used for biomolecules, this technique was never applied to the field of laboratory astrophysics, i.e. with samples at very low temperature. As a consequence, with a 3 hours delays between two clean ice samples, the signal optimisation has became a long task. For this reason, we chose light hydrocarbons such as methane (CH4) and ethane (C2H6), because of their relative low desorption temperatures. The sensitivity problem has been personally anticipated by attending the 60th ASMS conference on mass spectrometry (Vancouver, May 2012). This annual conference is one of the preeminent international meetings for the discussion of mass spectrometry instrumentation, techniques, and analytical tools. This meeting was therefore the unique opportunity for me to get external advices from experts on time-of-flight mass equipments. The benefits of that conference have triggered a drastic update of MATRIICES in a more compact way. The whole experimental setup had to be reconstructed (November 2012 - December 2012). Briefly, the substrate holder is now located very close by and between the ions extraction lenses, hence allowing the laser-desorption and the ionisation at the same spot. These two events were originally separated by 35-40 cm, i.e. a too long distance for an optimum molecular transmission. MATRIICES version 2 had to be first tested (January 2013) and then improved. During the very first tests, a spark in the electron gun has damaged the complete set of power supplies of the time-of-flight mass spectrometer. This has caused three months of delay. Because of the gain in sensitivity, a new substrate holder - prepared with an alternative gold coating technique - had to be purchased in order to get rid of chlorine peaks pollution. Finally, issues with opaque/leaking MgF2 windows have also further delayed our work. We have now succeeded in getting the first time-of-flight mass spectrum of laser-ablated photoproducts with now acceptable signal-to-noise ratio (November 2013). A technical review on MATRIICES version 2 has been accepted (September 2014) in the Review of Scientific Instruments: D. M. Paardekooper, J.-B. Bossa, K. Isokoski, and H. Linnartz. ”Laser desorption time-of-flight mass spectrometry of cryogenic ices”. My contribution in writing this paper was non-negligible, I started to write and sent a first draft beginning 2014. This has given the current PhD student Daniel Paardekooper a good starting material to complete the remaining sections. The paper reports two series of test experiments involving argon and methane pure ices. The series of LabVIEW/Matlab programs I wrote during the last years are still running and are updated when needed. They allow current and future experimentalists on the MATRIICES setup to acquire and process a large workflow of data in a simple and friendly way. The quantitative analysis that I developed (April 2014 - September 2014) is now fully operational and allows kinetic studies in the solid phase. Effective rate constants and branching ratios can now be determined for non polar ices. A first authorship paper is currently under review (submitted in January 2014 to Physical Chemistry Chemical Physics: “Methane ice photochemistry and kinetic study using laser desorption time-of-flight mass spectrometry at 20 K”). Astrochemical models use and often extrapolate gas phase reaction rate coefficients for the solid state. This study shows that this may lead to considerable errors. It also shows that laboratory kinetics studies in the solid phase are now possible and provide for the first time data highly needed in astrochemical models. These values are in strong demand by the astrophysical community and especially for modellers that simulate the gas-grain interaction.

In the meantime, I was the leader of the first porosity measurements project of porous H2O:CO2 ice mixtures. This side project was not initially specified in the NATURALISM proposal, but fits nicely within its main ideas. Porous ices are expected to increase the efficiency of solid state astrochemical processes since they are chemically more reactive than compact (non-porous) ices; they provide larger effective surface areas for catalysis, for the freeze-out of additional atoms and molecules, and for the trapping of volatiles. Understanding how an impurity (e.g. CO2) influences the H2O porosity during ice growth is therefore crucial for predicting the chemical evolution of interstellar ice analogues in different astronomical environments. We combined optical laser interference and extended effective medium approximations to measure the porosity of three astrophysically relevant ice mixtures: H2O:CO2=10:1, 4:1, and 2:1. Infrared spectroscopy was used as a benchmarking test of this new laboratory-based method. By independently monitoring the O-H dangling modes of the different water-rich ice mixtures, we confirmed the porosities predicted by the extended effective medium approximations. We also demonstrated that CO2 premixed with water in the gas phase does not significantly affect the ice morphology during omni-directional deposition, as long as the physical conditions favourable to segregation are not reached. We proposed a mechanism in which CO2 molecules diffuse on the surface of the growing ice sample prior to being incorporated into the bulk and then fill the pores partly or completely, depending on the relative abundance and the growth temperature. This project has involved two PhD students (Karoliina Isokoski and Daniel Paardekooper), two bachelor students (Ellen van der Linden and Thomas Triemstra), and one master student (Maelle Bonnin). I was in charge for the supervision in the laboratory as well as during the writing phase of the bachelor/master thesis. This project has also triggered a collaborative work with Dr. Stephanie Cazaux from the Kapteyn Astronomical Institute (Groningen, The Netherlands) and Prof. A.G.G.M Tielens from the Leiden Observatory, Leiden University (Leiden, The Netherlands). An article has been accepted (November 2013) to the Astronomy & Astrophysics journal: J.-B. Bossa, K. Isokoski, D. M. Paardekooper, M. Bonnin, E. P. van der Linden, T. Triemstra, S. Cazaux, A. G. G. M. Tielens and H. Linnartz, ”Porosity measurements of interstellar ice mixtures using optical laser interference and extended effective medium approximations”. Another article has been accepted (December 2013) to the Physical Chemistry Chemical Physics themed issue Astrochemistry of Dust, Ice and Gas: K. Isokoski, J.-B. Bossa, T. Triemstra, and H. Linnartz, ”Porosity and thermal collapse measurements of H2O, CH3OH, CO2, and H2O:CO2 ices”. Another article (October 2014) has been accepted to the Astronomy & Astrophysics journal: S. Cazaux, J.-B. Bossa, A.G.G.M. Tielens and H. Linnartz, “Pore evolution in interstellar ice analogues: simulating the effects of temperature increase”.

Infrared spectroscopy of the O-H dangling modes has up to now been the unique tool for characterising the morphology of ices in space. However, their remote detections are made difficult by the weak intensities, the overlaps with the strong O-H stretching modes of water, and the band shapes that change depending on the environment. An alternative probing tool is therefore mandatory to assess the question of the porosity of inter- and circumstellar ices. In September 2014, I started to train one bachelor student (Coen Fransen). We were trying to build a database of effective optical constants of inhomogeneous ice materials in the mid infrared, based on the extended Maxwell-Garnett and Bruggeman effective medium approximations (EMAs). These data will be very important for the interpretation of the mid-infrared spectra recorded both in the laboratory and in space. We would like to generate a set of effective optical constants in the mid-infrared by changing artificially the ice composition. For that purpose, we need to measure first several reference infrared spectra of pure, compact (i.e. non porous) and amorphous H2O and CH4 ices with known thicknesses. In a second step, we use a numerical method for the determination of the complex refractive index (n − ik) that uses the dispersion relations of Kramers-Kronig. Once n and k values of the reference ices are known for each wavenumber in the mid infrared, the extended EMAs can be applied to generate the database. The theories may have some limitations and we need to test them first in order to give error bars later on. As a check, and in order to quantify the limits of the theories, we are currently confronting the models with experimentally measured porous H2O:CH4 ice mixtures (with known compositions and known optical constants). After completion of this system and validation of both models, we would like to continue the project with other astrophysical relavant volatile molecules like CO, CO2, and NH3. The NIR and FIR regions could be also investigated but using a different spectrometer and/or optic systems. A collaboration with Dr. Belen Mate (IEM Madrid in Spain) and Prof. Sergio Pilling (UNIVAP in Brazil) on that matter has already started.

I have investigated the laboratory rotational spectrum of 1,2-propanediol in the 38-70 GHz, 200-230 GHz and 297-400 GHz region. This has been performed in March 2013 and November 2013 at the Cologne Laboratory Astrophysics Group (CLAG, University of Cologne, Germany). Diols in general and 1,2-propanediol in particular are key organic intermediates towards the formation of hydroxylated compounds and sugars in space. Investigating any interstellar complex molecules of the sugar family or their derivatives in astronomical environment is therefore of high importance for the RNA world and the question of the origin of life in the universe. Therefore, the study of 1,2-propanediol in Cologne matches one-to-one the third research training objectives of the NATURALISM proposal. I have identified more than two thousand rotational transitions for the three lowest conformers of 1,2-propanediol. More lines have to be interpreted in the 297-400 GHz region. The rotational spectra of the three conformers were predicted and fitted employing the SPFIT/SPCAT program package of H. M. Pickett. An article has been accepted (August 2014) to the Astronomy & Astrophysics journal: J.-B. Bossa, M. H. Ordu, H. S. P. Muller, F. Lewen, and S. Schlemmer, in preparation, ”Laboratory spectroscopy of 1,2-propanediol at millimeter and submillimeter wavelengths”. A collaborative work between Dr. Patrice Theule (Aix-Marseille University) and Prof. Stephan Schlemmer (University of Cologne) has started and my task was to help in designing, building and implementing a new experimental setup in Cologne that allows the detection of desorbing ice molecules (complex organic molecules) using a state-of-the-art chirped pulse Fourier transform microwave spectrometer.

I intensively promoted the Marie Skłodowska-Curie actions and the research performed during the NATURALISM project by giving talks and seminars in different international meetings and groups:

1. Icy Grain Chemistry for Formation of Complex Organic Molecules: From Molecular Clouds to Protoplanetary Disks, Comets and Meteorites, March 2015, Tokyo (Japan). Oral presentation.
2. Second Workshop on Experimental Laboratory Astrophysics, February 2015, Poipu, Kauai (USA). Invited speaker.
3. Colloque national bi-annuel du programme de Physique et Chimie du Milieu Interstellaire (PCMI) de l’INSU, October 2014, Rennes (France). Oral presentation.
4. LASSIE/YAM/DAN meeting, January 2014, Leiden (The Netherlands). Oral presentation.
5. 2nd Rencontres Societe Francaise d’Exobiologie (SFE), November 2012, Frejus (France). Invited speaker.

1. MONARIS laboratory, November 2014, Paris (France). Seminar.
2. W. M. Keck Laboratory in Astrochemistry, March 2014, Honolulu (USA). Seminar.
3. NASA Ames Research Center, March 2014, Mountain View (USA). Seminar.
4. California Institute of Technology, March 2014, Pasadena (USA). Seminar.
5. Jet Propulsion Laboratory, March 2014, Pasadena (USA). Seminar.
6. Institute of the Structure of Matter, February 2014, Madrid (Spain). Seminar.
7. Institut de Planetologie et d’Astrophysique de Grenoble, December 2013, Grenoble (France). Seminar.
8. Cologne Laboratory Astrophysics Group, March 2013, Cologne (Germany). Seminar.