Carbon ‘fluffy’ aggregates produced by helium–hydrocarbon high-pressure plasmas as analogues to interstellar dust

MNRAS 481, 2841–2850 (2018) doi:10.1093/mnras/sty2497 Advance Access publication 2018 September 13 Carbon ‘fluffy’ aggregates produced by helium–hydrocarbon high-pressure plasmas as analogues to interstellar dust Bianca Hodoroaba,1 Ioana Cristina Gerber,1 Delia Ciubotaru,1 Ilarion Mihaila,2 Marius Dobromir,3 Valentin Pohoata 1‹ and Ionut Topala1‹ 1 Iasi Plasma Advanced Research Center (IPARC), Faculty of Physics, Alexandru Ioan Cuza University of Iasi, Blvd. Carol I No. 11, Iasi 700506, Romania Center of Environmental Science Studies in the North-Eastern Development Region (CERNESIM), Alexandru Ioan Cuza University of Iasi, Blvd. Carol I No. 11, Iasi 700506, Romania 3 Research Department, Faculty of Physics, Alexandru Ioan Cuza University of Iasi, Blvd. Carol I No. 11, Iasi 700506, Romania 2 Integrated ABSTRACT This study provides an investigation on the deposition of carbon-related products on different substrates using a high power dielectric barrier discharge fed with helium and saturated hydrocarbon gases. The discharge has been characterized by means of voltage–current measurements, while the neutral species and the dissociation compounds accumulated in the plasma reactor have been analysed by Fourier-transform infrared (FTIR) spectroscopy. The properties of non-aromatic hydrogenated amorphous carbon (a–C:H) films and ‘fluffy’ carbon dust deposited on to various substrates are analysed by FTIR and X-ray photoelectron spectroscopy. The sp2 -hybridized fraction of a–C:H films is negligible while for the ‘fluffy’ dust, does not exceed 10 per cent. The microscopic appearance, the hierarchical organization, and the CH2 /CH3 ratio around 2 of the ‘fluffy’ dust analogue are results that supports the current understanding of cosmic carbon dust physico-chemical properties. Key words: astrochemistry – plasmas – dust, extinction – Galaxy: center – infrared: ISM. 1 I N T RO D U C T I O N Infrared emission and absorption spectroscopy or imaging, applied for the study of astronomical objects, revealed many insights into the organic chemistry in space. Many satellites, space telescopes, airborne telescopes, or ground based telescopes, were designed to host scientific instruments dedicated to specific wavelength ranges in near-, mid-, and far-infrared. The upcoming mission of NASA, ESA, and CSA, will carry the James Webb Space Telescope (JWST) on board and after the successful deployment of the infrared telescope, the scientific community will have access to new observational data, at high resolution, and may shed some new light on the hypothesis concerning the carbon compounds and their processing in space. On one hand, the presence of infrared emission bands with central wavelengths ranging from 3 to 12 μm (i.e. the so-called unidentified infrared bands) and related to carbon based molecules was detected for many astronomical objects. More specific, bands observed at 3.3, 6.2, 7.7, 8.6, and 11.3 μm, where initially associated with aromatic carbon vibrational features and led to the construction of polycyclic aromatic hydrocarbon (PAH) hypothesis in 1985 (Leger & Puget 1984; Allamandola, Tielens & Barker 1985). On the other hand, infrared absorption spectroscopy data  E-mail: (VP); (IT) toward various lines of sight, Galactic or extragalactic, provided enough data to understand that, under astronomical conditions, no unique chemical composition exists for the carbon dust. Instead, carrier families based on hydrogenated amorphous carbon (HAC or a–C:H), a hypothesis launched back in 1983 (Duley & Williams 1983), are considered to be responsible for the observations of specific environments, such as Galactic Center sources, dense clouds, or diffuse interstellar medium. Usually, a mixture of aliphatic (e.g. 3.4, 6.9, and 7.3 μm) and aromatic absorption features are observed, the aromatic to aliphatic ratio being function on the local thermodynamic parameters, radiation fields, and hydrogen content. In order to match the astronomical observations and to elucidate the source of the unidentified infrared bands, efforts have been made either to theoretically asses the carbon materials molecular structure and spectral properties (Peeters et al. 2002; Gray & Edmunds 2004; Cohen & Barlow 2005; Draine & Li 2008; Li & Draine 2008; Li & Draine 2012; Papoular 2013; Dhanoa & Rawlings 2014; Mauney & Lazzati 2016) or to design experiments for appropriate Earth based synthesis of analogue materials (Colangeli et al. 1997; Mennella, Brucato & Colangeli 2001a; Mennella et al. 2002a; Rotundi et al. 2002; Kovačević et al. 2005; Stefanović et al. 2005; Dartois et al. 2007; Stefanović et al. 2007; Pino et al. 2008; Biennier et al. 2009; Carpentier et al. 2012; Gadallah, Mutschke & Jäger 2013). In order to mimic the chemical and typological features of interplanetary and interstellar carbon dust, respectively, many synthesis  C 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Accepted 2018 September 8. Received 2018 September 7; in original form 2018 April 26 2842 B. Hodoroaba et al. MNRAS 481, 2841–2850 (2018) 2009; Vallade et al. 2014; Bazinette et al. 2016; Brunet et al. 2017, 2018), gas conversion (Eliasson, Liu & Kogelschatz 2000; Liu et al. 2001; Chiper et al. 2010; Obradović, Sretenović & Kuraica 2011; Schiorlin, Klink & Brandenburg 2016), decontamination, sterilization or inactivation (Heise et al. 2004; Eto et al. 2008; Chiper et al. 2011; Daeschlein et al. 2012; Cullen et al. 2018), and life science applications (Kuchenbecker et al. 2009; Rajasekaran et al. 2009; Brehmer et al. 2015; Rezaei, Shokri & Sharifian 2016; Ito et al. 2018). Nevertheless, some specific studies employing DBD, were performed in connection with space science (Wang et al. 2008; Thejaswini et al. 2011; Mihaila et al. 2016). Overall, the DBDs have attracted a significant research interest due to the flexibility to fit into more complex plasma diagnostics set-ups, their relatively standardized electrical excitation methods, their operation at atmospheric or sub-atmospheric pressure, the absence of ion etching processes, high collisionality and low Debye length, low ionization degree, and low gas temperature, the pulsed chemistry and the relatively high amount of reactive species generated per unit volume. In this work, we present a new laboratory-scale method to synthesize carbon dust analogues using the sub-atmospheric pressure plasma generated in DBD reactor fed with helium in mixture with several hydrocarbon gases (i.e. Cn H2n+2 , with n = 1–4). The plasma assisted hydrocarbon gas conversion was successfully proved by means of gas infrared spectroscopy, either into lower mass hydrocarbons, with the same general formula or new compounds, such as C2 H2 or C2 H4 . This is a new low temperature synthesis method for carbon dust analogues in form of both, films and dust products, no other experimental observations being available using a similar pl (...truncated)