Mid- and far-infrared absorption spectroscopy of

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Cyril Szopa. 1 ... VIMS instrument onboard Cassini is in agreement with an aerosol ... present some discrepancies with observations done by both by Cassini ...
Mid- and far-infrared absorption spectroscopy of Titan's aerosols analogues

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Author List

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Alexandre Giuliani3,4, Cyril Szopa1 , Carrie M. Anderson5, Jean- Jacques Correia1,

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Paul Dumas3 and Guy Cernogora1

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d'Alembert 78280 Guyancourt, France

Thomas Gautier1, Nathalie Carrasco1, Ahmed Mahjoub1, Sandrine Vinatier2,

LATMOS, Université Versailles St Quentin, UPMC Univ. Paris 06, CNRS, 11 Bvd

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LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195, Meudon, France

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Synchrotron SOLEIL, L’orme des Merisiers, BP 48, Saint Aubin, F-91192 Gif sur

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Yvette, France

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INRA, U1008 CEPIA, Rue de la Géraudière, F-44316 Nantes, France

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5

NASA Goddard Space Flight Center, Solar System Exploration Division, Greenbelt,

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MD 20771, United States

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Editorial correspondence to:

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Mr. Thomas Gautier LATMOS - University of Versailles St Quentin 11 Boulevard d'Alembert 78280 Guyancourt France Phone: (0033) 1 80 28 52 77 Fax: (0033) 1 80 28 52 90 E-mail: [email protected] Published in Icarus 221 (1) : 320-327

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Abstract

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In this work we present mid- and far-Infrared absorption spectra of Titan's aerosol

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analogues produced in the PAMPRE experimental setup. The evolution of the linear

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absorption coefficient ε (cm-1) is given as a function of the wavenumber.

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We provide a complete dataset regarding the influence that the concentration of

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methane vapor in the gas mixture has on the tholin spectra. Among other effects, the

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intensity of the 2900 cm-1 (3.4 µm) pattern (attributed to methyl stretching modes)

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increases when the methane concentration increases. More generally, tholins

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produced with low methane concentrations seem to be more amine based polymers,

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whereas tholins produced with higher methane concentrations contains more

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aliphatic carbon based structures.

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Moreover, it is shown that the position of the bands around 2900 cm -1 depends on

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the chemical environment of the methyl functional group. We conclude that the

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presence of these absorption bands in Titan's atmosphere, as measured with the

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VIMS instrument onboard Cassini is in agreement with an aerosol contribution.

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We also compare the far-infrared spectrum of tholin to spectra of Titan's aerosols

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derived from recent Cassini-CIRS observations displaying many similarities,

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particularly with absorption bands at 325 cm -1, 515 cm-1, and the methyl attributed

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1380 cm-1 and 1450 cm-1 bands.

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1 Introduction

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The atmosphere of Titan is mainly comprised of N2 and CH4. Organic chemical

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reactions are induced by solar irradiation and electrically charged particles

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accelerated in Saturn's magnetosphere (Sittler Jr et al. 2009). These reactions lead

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to the production of an opaque layer of organic solid aerosols in the atmosphere.

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These aerosols have a major impact on several parameters of Titan's atmosphere,

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such as the greenhouse effect (McKay et al. 1991), or condensation (Lavvas et al.

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2011). Studying Titan's aerosols is then of primary importance in order to understand

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both the physics and the chemistry of Titan's atmosphere.

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One possible and achievable method to study Titan’s aerosol is to produce and study

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laboratory analogues, coined "tholins". A discussion on the different experimental

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setups designed for such a purpose can be found in Cable et al. (2011). The

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properties of the produced tholins allow a better analysis and understanding of

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observational data of the atmosphere of Titan.

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However, it should be noted that, even if Titan's aerosol spectrum is better

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characterized at present by observations in the infrared spectral range thanks to the

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Cassini mission (Bellucci et al. 2009; Rannou et al. 2010; Vinatier et al. 2010;

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Anderson and Samuelson 2011; Vinatier et al. 2012) very few laboratory studies

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provide data on tholin in the extended far- to mid-infrared spectral domain. Up to

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now, the complex refractive indices of tholin published in Khare et al. (1984) were

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often used since they cover a large spectral range from 0.02 µm to 920 µm but

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present some discrepancies with observations done by both by Cassini Composite

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Infrared Spectrometer (CIRS) and Visible and Infrared Mapping Spectrometer (VIMS)

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instruments.

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Quirico et al. (2008) and Imanaka et al. (2004) have reported different absorption

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spectra on laboratory tholin, but only down to 500 cm-1. Very recently, Imanaka et al.

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(2012) derived the optical constants in the 400 - 4000 cm-1 range at three different

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pressures and with a given gas mixture (10% of CH4).

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Here we report new optical data, from the far-IR ranges down to 100 cm-1, a spectra

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region that was not studied since the Khare et al. (1984) work, to the mid-IR up to

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4000 cm-1. We performed experiments with four different initial gas mixtures. The

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tholins spectra are compared with those of Titan's aerosols recently acquired by the

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CIRS and VIMS instruments

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2 Experimental setup and protocol

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2.1 PAMPRE experiment

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Tholins were produced with the PAMPRE device (Szopa et al. 2006). PAMPRE is a

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low pressure radiofrequency capacitively coupled cold plasma (RF CCP) at

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13.56 MHz. RF CCP discharges are well-known for producing thin films on substrates

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and solid particles in the volume of reactive gas mixtures (Bouchoule 1999). The

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plasma discharge is generated from a gaseous mixture between a polarized

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electrode and a cylindrical grid grounded electrode confining the plasma (Alcouffe et

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al. 2010). The gas mixture is continuously injected through the meshed polarized

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electrode and pumped through a rotary valve vacuum pump. This discharge design

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carries out a uniform gas flow in the confining box where tholins are produced. The

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reactive gas mixture is at room temperature and can be adjusted with gas flow

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controller in order to introduce from 0% to 10% of CH4 in N2. Some chemical and

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physical properties of tholins produced in PAMPRE have already been investigated

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(Carrasco et al. 2009; Hadamcik et al. 2009; Pernot et al. 2010) as well as the gas

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phase chemistry leading to tholins formation (Sciamma-O'Brien et al. 2010; Gautier

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et al. 2011)

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2.2 Sample production

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The production was performed with a continuous 55 standard cubic centimeter

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(sccm) gas flow rate. In this work, samples were prepared with various N 2:CH4 gas

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mixtures including: 1%, 2%, 5% and 10% of methane. For the production of tholin,

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the power injected in the reactor was 30 W and the gas pressure 100 Pa. Substrates

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for thin film deposition were placed on the grounded electrode.

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Two types of substrates were used for tholin thin film deposition, MirrIRTM and silicon

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wafers.

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Tholin films on MirrIRTM substrates (Low-e microscope slides from Kevley

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Technologies, dim. 25mm x 75mm x 2mm) were used for acquiring data in the mid-IR

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frequency region, from 700 cm-1 up to 4000 cm-1 (14.2 µm to 2.5 µm).

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For the far-IR analyses, the thin films were deposited onto circular silicon wafers

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(1 cm diameter, 0.5 mm thickness). Thin films on silicon substrates allow for

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obtaining the spectra from 700 cm-1 down to 100 cm-1 (14.2 µm to 100 µm).

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Deposition of thin films on MirrIRTM substrates were achieved after 2 hour long

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experiments. Since the absorption of tholin is weak in the far-IR, productions of thin

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film on silicon were three hours long in order to increase the thickness of the sample.

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Three Si substrates were used in order to ensure film properties in the far-IR. For the

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mid-IR, only one MirrIRTM substrate was used, but as the substrate is large enough,

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three different areas were studied.

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2.3 Measurement of film thickness

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The thin film thicknesses were determined by spectroscopic ellipsometry. This

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technique is based on the measurement of the change of the light polarization upon

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light reflection on a sample (Fujiwara 2007). The reflected light from the sample

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surface is elliptically polarized.

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coefficients rp and rs, respectively for the light polarized perpendicularly and parallel

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to the incident plane is related to  and  parameters defined as:

, the ratio between the complex reflection

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()

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The ellipsometry parameters,  and, are related to the complex refractive indices of

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the substrate and of the different layer and to the thickness of the different sample

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layers. A model taking into account the optical properties and thickness of the

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different layers must established in order to deduce both the optical constants and

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the layer thickness. The method is described in details in Sciamma-O’Brien et al.

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(2012) and Mahjoub et al. (2012).

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We used an M-2000V spectroscopic ellipsometer from J.A. Woollam Co. The M-

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2000V is a rotating compensator ellipsometer with a CCD detector that measures all

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wavelengths simultaneously across the spectral range 370 nm to 1000 nm.

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The layer modeling was performed using the Complete-EASETM software (Complete

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EASETM Data Analysis Manual By J.A Woollam Co Inc. June 15, 2008).

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thicknesses measured for the different studied samples are presented in Table 1.

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Uncertainties on each sample are due to film non uniformities calculated by the

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Complete-EASETM software.

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(1)

The

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2.4 Far- and Mid-Infrared spectroscopy

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Spectra were recorded at the SMIS (Spectroscopy and Microscopy in the Infrared

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using Synchrotron) beamline of SOLEIL synchrotron radiation facility in France

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(Dumas et al. 2006). A NicPlan microscope was used coupled to a Nicolet Magna

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System 560 Fourier Transform Infrared (FTIR) spectrometer. The IR sources utilized

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for the present work were the synchrotron radiation for mid-IR and the internal Globar

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source for the far-IR. The detectors were either the Mercury-Cadmnium-Telluride

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(MCT) detector of the microscope (mid-IR), or a silicon doped bolometer from

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Infrared Laboratories, cooled down to 4.2 K with liquid helium (far-IR). Analyses were

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performed in transmission mode.

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To ensure the repeatability of the measurements in the far-IR, thin film analyses were

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performed on the three samples, three times each, i.e. 9 sample spectra were taken

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in the far-IR range. We also collected three points of reference on two tholin-free

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silicon wafers to be used as the substrate reference spectra.

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In the mid-IR, 6 spectra were collected on the thin film deposited on MirrIRTM

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substrates and blanks were performed on a tholin free substrate.

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Spectra were recorded at a spectral resolution of 4 cm-1 after co-adding 512 scans at

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a Michelson mirror velocity of 1.26 cm.s-1.

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3 Results

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All the spectra presented hereafter correspond to average spectra. Error bars are the

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standard deviations of measurements performed on samples produced in the same

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conditions.

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3.1 Thickness calibration and linear absorption coefficient determination

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Figure 1 presents an absorbance uncalibrated spectrum of tholin (produced with 5%

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of methane) from 100 cm-1 to 3500 cm-1. The spectrum is made of two parts:

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1) below 700 cm-1 corresponding to the measurement of tholin film deposited onto

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silicon,

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2) above 700 cm-1 corresponding to the measurement of film deposited on MirrIRTM

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substrates.

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The non continuity of the spectrum at 700 cm-1 is due to the difference of thickness of

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the two samples.

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Figure 1

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In order to consider the effect of the film thickness, the linear absorption coefficient, ,

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should be used. The calculation of this coefficient requires the sample thickness, d.

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The reflected signal was measured and found negligible compared to the transmitted

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contribution. The linear absorption coefficient,  (cm-1), can thus be defined using the

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Beer-Lambert law: (

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)

(2)

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where d is the sample thickness, It is the intensity of the transmitted signal, I0 the

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incident intensity.

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Note that the absorption coefficient  is a function of the absorption cross-section, σ,

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and the density of absorbent molecules, [n], not measurable here: (3)

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Absorbance, A, being defined as:

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(

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The average thickness of thin films on silicon wafers was measured to be

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1300 ±45 nm.

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The multilayer structure of MirrIRTM substrates does not allow ellipsometric

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measurements. The thickness of the film was thus estimated by fitting both parts of

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the spectrum depicted in Fig.1 in order to get continuity of ε on the whole spectral

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range. Using this method, the estimated thickness of the tholin films on MirrIRTM

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substrates was 580 ± 50 nm. In order to validate this thickness estimation, tholin films

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have been deposited on CaF2 substrates of the same thickness that MirrIRTM

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substrates and during the same duration. Measurement on the film deposited on pure

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CaF2 gives a thickness of 550 ± 25 nm. This value is in agreement with the thickness

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estimated by fitting the spectra. We thus consider that the thicknesses of films on

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pure CaF2 provides a good estimation of the thickness of the film on MirrIR TM

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substrates, and we used this method to infer the thickness of other films deposited on

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MirrIRTM as shown in Table 1.

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Sample

Injected % CH4

Deposit duration

Thickness (nm) 1250 ± 40

Si n°1 3h

Si n°2

1340 ± 40

5% Si n°3

1300 ± 40

CaF2

550 ± 25

CaF2

1%

420 ± 10 2h

CaF2

2%

490 ± 15

CaF2

10%

580 ± 25

200 201

Table 1: Experimental conditions and thickness of the different studied samples

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The difference of thickness between films on silicon and on CaF2 comes from both

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their respective production times and the dielectric properties of the substrates, as

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discussed in Mahjoub et al. (2012). Note that since both Si and MirrIRTM substrates

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are reflective mirrors, the optical path length during the absorption is equal to two

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times the film thickness.

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3.2 Band assignment

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In the following, absorption spectra are measured from tholins produced with 5% CH 4

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in N2 as the gas mixture. It has been shown that in the PAMPRE experiment, 5% of

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methane injected corresponds to approximately 2% in steady state conditions

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(Sciamma-O'Brien et al. 2010). The wavenumber dependence of the linear

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absorption coefficient  is shown in Fig. 2.

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Figure 2

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The red curve corresponds to the average spectrum, and the pink envelope includes

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both the film thickness uncertainty and the statistical 1 σ standard deviation errors

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from the different measurements.

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The larger uncertainty observed at shorter wavenumbers (around 700 cm-1) is due to

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both the limitations of the MirrIRTM substrates and the detection efficiency of the MCT

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detector.

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Silicon has a strong absorption band in the 590 cm-1 - 620 cm-1 range which prevents

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the determination of the linear absorption coefficient in this range when the spectra is

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divided by blank spectra. The spectrum is therefore interpolated between 590 and

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620 cm-1.

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The spectrum shown in Fig. 2 depicts common tholin absorption features in the mid-

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IR, described for example in Coll et al. (1999), Imanaka et al. (2004), Quirico et al.

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(2008) or Imanaka et al. (2012). Broad and intense bands at 3200 cm-1 (3.13 µm)

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and 3330 cm-1 (3.00 µm) are due to primary -NH and secondary amines -NH2. The

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2880 cm-1 (3.47 µm) band is attributed to –CH3 symetric stretching. The 2930 cm-1 Page 11

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(3.41 µm) and the 2960 cm-1 (3.38 µm) bands are attributed to -CH2 asymmetric

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stretching and –CH3 asymmetric stretching.

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The three bands observed at 2140 cm-1 (4.67 µm), 2175 cm-1 (4.60 µm) and

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2250 cm-1 (4.44 µm) can be attributed to nitriles -C≡N, isocyanides -N≡C or

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carbodiimide –N=C=N- stretching modes.

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The 1560 cm-1 and 1630 cm-1 bands cannot be assigned unambiguously. They can

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correspond to several possible functional groups, such as aromatic or aliphatic –NH2,

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C=N double bonds, C=C double bonds, aromatics or heteroaromatics (bearing

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nitrogen). This precludes a precise assignment of these bands.

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At lower wavenumbers, we observe two bands at 1380 cm-1 (7.2 µm) and 1450 cm-1

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(6.9 µm). As suggested in Vinatier et al. 2012, the 1450 cm -1 band is probably a

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contribution of asymmetric C-H bending of CH3 and scissor in plane bending of C-H

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in CH2, and the 1380 cm-1 band the symmetric bending of C-H in CH3.

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Several absorption features are also visible in the far-IR frequency range, as shown

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in Fig. 7. First a broad feature extends from 400 to 600 cm-1, which might be due to

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amorphous carbon nitride (Rodil et al. 2001; Quirico et al. 2008). But as said in

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Quirico et al. (2008) "it cannot be simply interpreted in terms of wagging or skeletal

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modes of simple molecules, but rather as lattice vibrations within the covalent solids".

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Thus, this band is left unassigned.

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The band observed at 690 cm-1 (14.49 µm), could possibly be attributed to C-H

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bending out of plane in carbon-carbon double bonds (usually 665 to 730 cm -1) or to

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ring out of plane deformation vibration in aromatics (usually 670-720 cm-1).

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Absorption due to aromatic rings deformations are known to be more intense than

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absorption due to C-H out of plane bending mode. Further analysis on the aliphatic

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and aromatic content in tholin could confirm a preferential assignement of theses

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bands to aromatics ring deformations.

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Bands are also visible at 515 cm-1 (19.42 µm), 324 cm-1 (30.86 µm), 255 cm-1

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(39.22 µm) and 170 cm-1 (58.82 µm). However, their assignements are not obvious

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because of a lack of litterature data for these weak bands.

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3.3 Impact of the percentage of methane on tholin spectra

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We studied the influence of the methane concentration of the reactive gas mixture in

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the PAMPRE experiment on tholin spectra. Figure 3 presents ε as a function of the

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wavenumber for tholins produced with 1%, 2%, 5% and 10% of methane in the gas

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mixture injected. Sciamma-O'Brien et al. (2010) have shown that these injected ratios

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respectively correspond to ~0.2%, ~0.5%, ~2%, ~5% of methane in the steady state

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conditions of the reactive plasma. 3500 3000

-1  (cm )

2500

1% 2% 5% 10%

2000 1500 1000 500 0

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1500

2000

2500

3000

3500

 (cm-1)

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Figure 3

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When comparing the spectra presented in Fig. 3, the most striking point is a growth in

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intensity of the amine bands at 3200 cm-1 and 3330 cm-1 when the concentration of

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methane decreases in the initial gas mixture. These bands are obvious when 1% of Page 13

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methane is injected, whereas they are quite low in the spectrum when 10% of

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methane is used. This is consistent with the elemental analysis presented in

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Sciamma-O'Brien et al. (2010), showing an increase of the nitrogen content in tholin

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for the lower methane concentrations. Nevertheless, in spite of the fact that amines

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were recently confirmed to be functional groups present in our tholin with solid

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nuclear magnetic resonance (Derenne et al. 2012), their predominance was not

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expected considering the unsaturation supported by nitrogen reported in previous

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studies performed both on the gas phase (Coll P et al. 1999; Gautier et al. 2011) and

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on tholin (Somogyi et al. 2005; Carrasco et al. 2009; Pernot et al. 2010).

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This increase of the amine bands is correlated to the decrease of the intensity of the

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2900 cm-1 pattern, attributed to aliphatic methyl. For the lowest methane

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concentration (1%) the 2900 cm-1 pattern is very weak, whereas for the highest

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methane concentration studied (10%) it is one of the strongest absorption bands in

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the spectra. The increase in the strength of these bands suggests that the amount of

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aliphatic methyl increases in tholin when they are produced with a higher methane

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percentage.

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Another variation observed on the spectra presented in Fig. 3 is the progressive

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vanishing of the band at 2140 cm-1 compared to the 2175 cm-1 band. As said in

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section 3.2, these bands and the one at 2250 cm -1 are due to stretching modes of

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several kinds of carbon-nitrogen triple bonds. Their exact attribution is still unclear,

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but the 2140 cm-1 band decreases with respect to the 2175 cm-1 band when the

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methane percentage increases. This is consistent with the increase of the saturated

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aliphatic carbons, also observed on the 2900 cm -1 pattern. The 2140 cm-1 and the

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2175 cm-1 bands could respectively be attributed to unsaturated and saturated

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nitriles. Page 14

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Another visible effect visible in Fig. 3 is the plummeting of the band at 1560 cm-1

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respective to the 1630 cm-1 band. Indeed for low methane concentrations, the

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1560 cm-1 band is as intense as the one at 1630 cm-1, whereas it is almost

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undetectable in the spectra of tholin produced with 10% methane. This band is not

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clearly identified (cf. part 3.2), but may involve nitrogen, such as cyclic or aliphatic

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amines (c-NHx or n-NHx), C=N or heteroaromatics, since this band intensity

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decreases with the decrease of the N2 concentration in the gas mixture introduced in

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the experiment.

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Finally, the relative increase of the absorption bands at 1450 cm-1 and 1380 cm-1 with

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increasing CH4 percentage is also noticeable in Fig.3, as is the growth of broad low

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intensity bands centered at 1230 cm-1 and 1140 cm-1. These bands are possibly due

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to carbon-carbon or carbon-hydrogen bonds.

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All these band variations enforce the notion that the amount of molecules with

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hydrocarbon skeleton in tholin increases with the methane concentration in the

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reactive gas mixture, whereas tholins produced with low methane concentrations

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seem to be based on amine rich polymers.

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4 Comparison with Cassini CIRS and VIMS observations

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Figure 4 shows tholin absorption spectrum in the far- and mid-IR spectral range

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(black curve). Also plotted in this figure is the tholin spectrum of Khare et al. (1984),

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in blue. The CIRS and VIMS spectra of Titan's aerosols in the mid- and far-IR are

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plotted in red (Anderson and Samuelson 2011; Vinatier et al. 2012; Bellucci et al.

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2009; Kim et al. 2011).

Page 15

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In order to compare the tholin absorption spectra obtained with different conditions, a

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normalization coefficient was applied to the spectra. The Khare et al. (1984)

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spectrum was normalized to the maximum intensity of the entire tholin spectrum (i.e.

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the 1560 cm-1 band). VIMS and CIRS spectra were normalized to the maximum of

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our spectrum (i.e. the CIRS spectrum was normalized at the 1450 cm -1 band, and the

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VIMS spectrum at the 2930 cm-1 band).

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Figure 4

325 326

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The analysis of Fig. 4 is discussed in the mid- and far-IR spectral regimes: a) Mid-Infrared

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The 2900 cm-1 (3.4µm) pattern, emphasized in Fig. 5, is in general agreement with

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the feature observed by VIMS (Bellucci et al. 2009; Rannou et al. 2010). This pattern

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is not present in the spectrum presented by Khare et al. (1984), produced with a

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10% initial methane concentration. But in our set up, the intensities of these bands

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are maximum with 10% methane introduced in the experiment (cf. Fig. 3), and almost

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as intense as the feature at 1500 cm-1.

Page 16

1 Tholins 5% (this work) VIMS spectrum (adapted from Kim et al. 2011) Data from Khare et al. 1984 (adapted from Quirico et al. 2008)

Absorbance (a.u.)

0.8

0.6

0.4

0.2

0 2700

2800

2900

3000

3100

3200

3300

3400

3500

334

 (cm-1)

335

Figure 5

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A possible explanation is as follows. Despite the same initial methane concentration

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between our work and Khare et al. (1984), the effective consumption of methane

338

might be different. Indeed, the residual methane concentration in a steady state in

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the plasma is quite different than from the initial one as shown in Sciamma-O'Brien et

340

al. (2010). And yet, as shown in section 3.3, the intensity of the 2900 cm-1 pattern is

341

highly correlated with the methane concentration. However, it must be underlined that

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other plasma parameters can influence the chemical composition of tholins in

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addition to the injected amount of CH4, such as the working pressure as shown in

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Imanaka et al. (2012).

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Figure 5 compares the shape of the 2900 cm-1 feature between tholin spectrum and

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the derived aerosol spectrum from VIMS observations (extracted from Kim et al.

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2011).

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Among the five main bands observed in the VIMS data (2885 cm-1, 2930 cm-1,

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2965cm-1, 3000 cm-1 and 3030 cm-1) two are not found in our tholin spectrum: the

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3000 cm-1 band, attributed to CH3CN ice by Kim et al. (2011), and the weak 3030 cm-

351

1

band. The spectral position of the 2930 cm-1 band (attributed to n-CH2 assymetric

Page 17

352

stretching) in our data corresponds exactly to one of the absorption bands detected

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with VIMS in Titan's atmosphere. For the 2885 cm -1 and 2965 cm-1 bands, we

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assume they are actually the same as the bands detected in tholin respectively at

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2880 cm-1 and 2960 cm-1 and attributed to CH3 stretching modes. Indeed infrared

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spectra of solids can present slight frequency shifts due to the chemical environment

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for the functional group generating the absorption band (see Fig.6) ("Infrared

358

Spectra" by NIST Mass Spec Data Center, S.E. Stein, director). Particularly, a shift of

359

a few wavenumbers toward higher wavenumbers may arise from the influence of

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aromatics on the methyl vibration. In the same way, the chemical environment of the

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methyl group responsible for this pattern has a major impact on the presence of the

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bands shown in Fig. 6. In this figure the impact of the insertion of an heteroatom in

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the cycle of toluene on the 2900 cm-1 pattern is observed.

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365 366

Figure 6

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The red line represents the spectrum of a methyl group bonded to a pure carbon

368

aromatic ring (toluene), whereas green and blue lines are spectra with the methyl

Page 18

369

group bonded to an heteroaromatic ring (here pyridine) in two different positions. It is

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clear from Fig. 6 that the composition of the chemical environment of the methyl

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group (and even the position of the nitrogen in the cycle comparing blue and green

372

spectra) has a major influence on the shape of the spectra, and it can induce both

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band intensity changing and band position shifting.

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The fact that our tholin bands do not perfectly fit the bands seen with VIMS still does

375

not imply that these bands in Titan's atmosphere cannot come from aerosols

376

absorption. But this might suggest that the chemical environment of the methyl

377

functional groups in Titan's aerosols are, even slightly, different from the obsevred

378

features in our tholin.

379

In Kim et al. (2011), the authors proposed that the 2900 cm -1 absorption pattern

380

observed by VIMS could be due to a mixing of different ices (C2H6, CH4, CH3CN,

381

C5H12, C6H12). As shown in this work, tholins present an intense absorption at this

382

wavenumber. We thus suggest that aerosol absorption can explain the VIMS

383

observations better than ices, and in agreement with Rannou et al. (2010) and

384

Bellucci et al. (2009). Moreover, the condensation of these ices seems unlikely

385

considering the SVP curves of these compounds (Linstrom P.J. and W.G. Mallard,

386

Eds., NIST Chemistry WebBook, NIST Standard Reference Database Number 69) at

387

the altitude of the observation since Fulchignoni et al. (2005) measured a

388

temperature of about 180 K at 300 km, with a pressure of about 10 Pa.

389

b) Far Infrared

390

A close up of Fig. 4 in the 100 cm-1 – 1500 cm-1 wavenumber range (100 µm- 6.6 µm

391

wavelength range) is given in Fig. 7. In this range, our tholin spectrum is compared

392

with the absorption spectra attributed to Titan's aerosols derived from CIRS

Page 19

393

observations (Vinatier et al. 2010; Anderson and Samuelson 2011; Vinatier et al.

394

2012). 1

Absorbance (a.u.)

0.8

This work CIRS spectrum (adapted from Anderson & Samuelson 2011) CIRS spectrum (adapted from Vinatier et al. 2011) Data from Khare et al. 1984 (adapted from Quirico et al. 2008)

1450

1380

0.6

0.4

0.2

0 0

170

515

255

690

324

500

1000

1500

-1

395

 (cm )

396

Figure 7

397

In this frequency range, two absorption bands are clearly present in both the CIRS

398

and the tholin spectrum, the band at 1450 cm -1 (6.9 µm), slightly left-shifted in tholin

399

spectrum compared to observations, and the band at 1380 cm-1 (7.2 µm) which is

400

exactly co-located in both spectra. A third band is visible arround 1320 cm -1 (7.6 µm)

401

in the CIRS spectrum but not in our tholin spectrum (or really weak) and may be due

402

to an overtone of the C-H bending in ≡C-H group signature observed at 630 cm -1

403

from CIRS. This absence could mean that tholins produced with PAMPRE contain

404

less alkynes than the Titan aerosols.

405

Furthermore, as illustrated in Fig. 7, the broad band visible around 600 cm -1 in our

406

spectrum is also present in other laboratory tholins spectra (e.g. Imanaka et al. 2004;

407

Quirico et al. 2008; Imanaka et al. 2012), but is not present in Cassini-CIRS spectra.

408

As discussed in part 3.2, this feature is speculated to arise from the lattice vibration in

409

solids (Rodil et al. 2001). This would mean that this band could only originate from

410

large scale solid material such as laboratory tholin film, whereas this band could not

Page 20

411

be observed on suspended single particles (free and not assembled), such as

412

aerosols in Titan's atmosphere. We suggest that obtaining the spectra of tholins in

413

suspension would be a good way to validate this hypothesis.

414

Finally, spectrum of our tholin exhibit a few features in the far-IR below 700 cm-1,

415

(14.3 µm). Some of these features, especially the bands at 325 cm -1 (30.77 µm) and

416

513 cm-1 (19.49 µm) are also visible in CIRS spectra (Anderson and Samuelson

417

2011). Up to now these bands are attributed to noise in CIRS spectrum.

418

The fact that these two bands are also present in our tholin spectrum could tend to

419

confirm that these weak absorption bands visible in CIRS spectrum were not noise

420

but were due to aerosols. Further investigation on this spectral range of observations

421

of Titan's atmosphere could confirm such hypothesis.

422

5 Conclusion

423

In this work we present a study of the infrared absorption properties of tholins

424

produced with the PAMPRE plasma device for several gas mixtures.

425

This work provides the wavenumber dependence of the linear absorption coefficient

426

ε, from the far-IR (100 cm-1) to the mid-IR (4000 cm-1) range.

427

We also show the way in which the percentage of methane in the experiment directly

428

impacts the spectrum of tholins in the mid-IR. This influence is maximum on the

429

amine bands at 3330 cm-1 and 3200 cm-1, and also visible on the 2900 cm-1 pattern

430

due to aliphatic methyl, and in the 2200 cm-1 - 2400 cm-1 and 1300 cm-1 - 1650 cm-1

431

ranges.

Page 21

432

Furthermore, some comparisons are performed between different tholins material

433

and data derived from Cassini CIRS and VIMS observations. The 2900 cm -1 pattern

434

is in relatively good agreement with the Cassini-VIMS spectra obtained in Titan

435

atmosphere and possibly attributed to aerosols.

436

The high intensity of these bands in PAMPRE tholin spectra supports the hypothesis

437

that aerosols are the main contributors to the 2900 cm-1 (3.4 µm) absorption pattern

438

in Titan's atmosphere.

439

Comparing tholin spectrum with spectra obtained with Cassini-CIRS in the far-IR

440

frequency range, we see that the two spectra are in good agreement with many

441

absorption bands, especially at 1450 cm-1, 1380 cm-1, 515 cm-1 and 325 cm-1,

442

reinforcing the detection of these bands from CIRS.

443

Furthermore, the mid- and far-IR data provided in this work could also be used for

444

comparison to other astronomical environment where tholin material is supposed to

445

be relevant, such as cometary ices, Triton and TNO surfaces or diffuse interstellar

446

medium (Gradie and Veverka 1980; Pendleton and Allamandola 2002; Dotto et al.

447

2003).

448

6 Acknowledgments

449

This work was financially supported by CNRS (PNP, ANR-09-JCJC-0038 contract).

450

All the PAMPRE team gratefully thanks the SMIS beam line team for their help and

451

contribution and SOLEIL synchrotron facility for accepting and supporting the

452

project.n° 20100103.

Page 22

453

7 Figure Caption

454

Figure 1: Reconstructed experimental spectrum of tholin produced in PAMPRE with

455

5% of methane before calibration of the sample thickness.

456

Figure 2:

457

PAMPRE with 5% of methane. Average spectrum is given by the red curve. Pink

458

envelopes indicate this standard deviation (1 σ) of the spectra, representing the

459

variation of the spectrum from one measure to another on the same sample. Possible

460

attributions are given for major bands of the spectrum. n-X and c-X mean that the

461

functional group X is attached to respectively an aliphatic or an aromatic skeleton.

462

Figure 3: Absorption spectra of tholins in the mid-Infrared with different initial CH4

463

concentrations in the gas mixture (1% in blue, 2% in green, 5% in red and 10% in

464

cyan).

465

Figure 4: Absorbance spectrum from far-IR to mid-IR of PAMPRE tholin made with

466

5% of CH4 (black curve) compared to Quirico et al. 2008 tholin spectra (extracted

467

from

468

Anderson and Samuelson 2011 and Vinatier et al. 2011) and Cassini-VIMS spectra

469

(red dots, from Kim et al. 2011).

470

Figure 5: Close up of Fig. 3 on the 2900 cm -1 pattern. Black line represents our tholin

471

5% spectrum. Red dots are the spectrum derived from observations with Cassini-

472

VIMS attributed to Titan's aerosols (extracted from Kim et al. 2011). The blue line

473

represent tholin spectra reconstituted from Khare et al. 1984 data (from Quirico et al.

474

2008). Black vertical dashed lines represent bands seen by VIMS, respectively from

475

left to right: 2885 cm-1, 2930cm-1, 2965 cm-1, 3000 cm-1, 3030 cm-1.

Evolution of  from the far- to the mid-Infrared for tholin produced in

Khare et al. 1984 tholin, blue), Cassini-CIRS observations (red line, from

Page 23

476

Figure 6: Infrared spectra of toluene (red), 2-methylpyridine (blue) and 3-

477

methylpyridine (green) extracted from the NIST database.

478

Figure 7: Close up of Fig.3 in the far-IR range. Black line represents our tholin 5%

479

spectrum. Titan's aerosols spectra derived from observations with Cassini CIRS

480

(Anderson and Samuelson 2011; Vinatier et al. 2011) are plotted in red. The blue line

481

represents tholin spectra reconstituted from Khare et al. 1984 data (from Quirico et

482

al. 2008).

Page 24

483

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