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22nd International Symposium on Plasma Chemistry July 5-10, 2015; Antwerp, Belgium Considerations on the use of atmospheric pressure plasma to generate complex molecular environments with relevance for molecular astrophysics I. Topala, R. Jijie, A.V. Nastuta, I. Mihaila, I.A. Rusu and V. Pohoata 1 Iasi Plasma Advanced Research Center (IPARC), Faculty of Physics, Alexandru Ioan Cuza University of Iasi, Blvd. Carol I No. 11, 700506 Iasi, Romania Abstract: Atmospheric pressure plasmas devices are suitable experimental solutions to generate pulsed molecular environments with various applications. We will discuss here details related to the chemical mechanism in DBD discharges related to He+H 2 plasma processing of hydrocarbons in order to simulate the chemical environment in hot cores. Molecular beam mass spectrometry is employed to characterize the chemical environment and to compare experimental data with telescope observations. Keywords: atmospheric pressure plasma, mass spectrometry, molecular astrophysics 1. Introduction Interstellar medium represents a complex architecture, with a multiple phase structure. Among these phases, molecular clouds are of particular interest, even they represent less than 1% of total medium. The matter present in the interstellar medium is represented mainly by atoms and molecules; a small fraction is represents by carbon and silicon based grains. Biogenic elements (i.e. O, C and N) abundance is around 0.1%, while H and He represents more than 99% of atomic matter [1]. Young stellar objects can be detected within giant molecular clouds and some of these objects are represented by ambient gas at average temperatures of 300 K, known as hot cores. These are massive regions of dust and gas (containing a large fraction of molecules), with a mass range between 10 to 3000 solar masses and a strong emission in far infrared range. The molecular hydrogen density in central region of hot cores reaches to 107 cm-3 and the average temperature in the range 100300 K [2]. In contrast, most of the interstellar medium volume show high values of temperature (T > 103 K) and in consequence does meet criteria for molecular synthesis. Around 75 years ago, CH was first molecule discovered in space. Since then efforts have been made to identify more and more classes of molecules in interstellar space or at the surface of various astronomical objects and this campaign ended with more than 150 interstellar and circumstellar molecules, with an increase of the molecular mass and complexity (bond angles, length and atomic partners) [3]. Scientific data allowing new molecules discovery are coming mainly from telescope observations (i.e. rotational transitions data). It is of high interest to cross check these data with laboratory experiments results: plasma experiments, crossed molecular beams, selected ion flow tube mass spectrometry, UV processing or atomic/ion bombardment of interstellar ice analogues. From all these techniques, laboratory plasma experiments are valuable resources to study complex molecular environments and relatively not expensive experimental techniques [4]. Many results, in specific P-I-2-68 plasma conditions and experimental arrangements have shown that plasma can be used for simulations solid state astrochemical processes, Titan tholin formation, laboratory mimicking the interstellar dust formation and charging, synthesis of enol and keto species [5-7]. Our approach to generate complex chemical systems with relevance for the ongoing processes in hot molecular cores, is to use a non-equilibrium plasma at atmospheric pressure. The discharge is feed with a complex mixture of gases: helium – hydrogen – hydrocarbons (methane, ethane, propane, butane). Molecular beam mass spectrometry is used to detect and identify the chemical population generated in plasma volume. 2. Experimental A dielectric barrier discharge reactor was designed and constructed for this study in order to be coupled with the molecular beam mass spectrometer (HPR-60, Hiden Analytical). Two rectangular electrodes, supported by 1 mm thick glass discs and 0.74 mm gap, are used to generate the plasma in controlled gas mixture conditions, in a volume of about 2.32 cm3. The electrode arrangement is hosted by a stainless steel reactor. Helium flows between discharge electrodes with a flow rate of 2.45 L/min, while hydrogen is mixed at a constant flow rate of 1 mL/min. Four types of hydrocarbons were also independently introduced inside the reactor, with flow rates of 4.25 mL/min. High voltage sin wave is used to power one electrode, while the other is grounded (specific excitation conditions are: 2.45 – 2.6 kV amplitude, 1 kHz frequency). Multiple electrical breakdown occurs on every increasing and decreasing edge of the high voltage wave. This results in individual current pulses (multi peak discharge), with average amplitude of 68 - 80 mA. A quadrupole-based system was used for this purpose (HPR-60 MBMS, from Hiden Analytical Ltd), with 2500 amu upper mass range. The sampling system consists of three differentially pumped stages, separated by aligned skimmer cones. Neutral atomic or molecular gas population is detected in real time by the molecular 1 beam mass spectrometer. Moreover, positive and negative ion population is sampled from discharge volume and monitored during discharge operation. All mass spectrometry data presented in this work are time averaged, showing the overall effect of plasma on chemical synthesis and clustering effects. 3. Results and discussions The capacity of atmospheric pressure plasma generated based on dielectric barrier principle to generate transitory molecular populations with relevance for molecular astrophysics of hot cores was assessed by molecular beam mass spectrometry analysis. Fig. 1 shows typical data obtained for helium – hydrogen – hydrocarbons plasmas at atmospheric pressure, for both negative and positive ion populations. Fig. 1. Positive and negative ion mass spectra for various helium – hydrogen – hydrocarbons plasmas at atmospheric pressure. RGA monitoring of the neutral gas was also considered during our studies. Nevertheless data are not shown here due to possible interferences of ionization regions: plasma electrons and spectrometer electron gun. Ion population have a strong influence on plasma chemistry, by polymerization, recombination or clustering processes. Hydrocarbons molecule mass was found to influence the overall mass spectrum of the plasma, for both positive and negative ion population. For example, the positive ion spectra are dominated by grouped mass signals around C n H 2n-1 , C n H 2n , C n H 2n+1 masses (n up 8). On the other hand negative ion spectra are more complex and spread to higher masses. The population appears to be dominated by water clusters around negative core ions with general formula A-(H 2 O) n (n up to 26), with A 2 P-I-2-68 represented in various fractions by HO-, O 2 -, HO 2 -, NO 2 -, NO 3 -, CO 3 -. 4. Conclusions The atmospheric pressure plasma generated in helium – hydrogen – hydrocarbons represents a good solution to non-equilibrium chemical population, with a representative fraction of molecular clusters. Relatively high mass molecules are dissociated and recombination phenomena lead to many types of molecular families, some of them being also identified in molecular hot cores (i.e. hydrocarbon radicals or complex hydrocarbons). 5. References [1] D. Wooden, Composition and evolution of interstellar clouds. In Comets II, M. C. Festou, H. U. Keller, and H. A. Weaver (eds.), University of Arizona Press, 33‐66 (2004) [2] R. Rolffs, Structure of Hot Molecular Cores. PhD thesis, Köln University (2011) [3] J. Nuth et al., Chemical Processes in the Interstellar Medium: Source of the Gas and Dust in the Primitive Solar Nebula. In Meteorites and the Early Solar System II, D. S. Lauretta and H. Y. McSween Jr. (eds.), University of Arizona Press, 147‐167 (2006) [4] D. Schram, Astro chemistry – Plasma chemistry. Some plasma aspects: processes and questions, Annual meeting of the Astrophysical Chemistry Group of the RSC and RAS on progress in astrochemistry, intro 2, (2011) [5] H. Thejaswini et al., Plasma chemical reactions in C 2 H 2 /N 2 , C 2 H 4 /N 2 , and C 2 H 6 /N 2 gas mixtures of a laboratory dielectric barrier discharge. Advances in Space Research 48, 857–861, (2011) [6] W. Goedheer, Nucleation in plasmas, charge and dynamics of dust, Annual meeting of the Astrophysical Chemistry Group of the RSC and RAS on progress in astrochemistry, inv 4, (2011) [7] J. Wang, et al. Interstellar enols are formed in plasma discharges of alcohols. The Astrophysical Journal, 676, 416‐419, (2008) Acknowledgments: This work was supported by Romanian Space Agency (ROSA) under the project STAR CDI ID 349/2014-2016. P-I-2-68 3