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1
Volatile products controlling Titan’s tholins produ ction 1
Nathalie Carrasco1*, Thomas Gautier1, Et-touhami Es-sebbar1, Pascal Pernot2, Guy 2
Cernogora1 3
4
1Laboratoire Atmosphères, Milieux, Observations Spatiales, Université de Versailles Saint-5
Quentin, UMR 8190, 78280 Guyancourt, France 6
2Laboratoire de Chimie Physique, UMR 8000, CNRS, Univ Paris-Sud, 91405 Orsay cedex, 7
France 8
* Electronic Address: [email protected] 9
Abstract 10
A quantitative agreement between nitrile relative abundances and Titan’s atmospheric 11
composition was recently shown with a reactor simulating the global chemistry occurring in 12
Titan’s atmosphere [Gautier et al. (2011) Icarus, 213: 625]. Here we present a complementary 13
study on the same reactor using an in-situ diagnostic of the gas phase composition. Various 14
initial N2-CH4 gas mixtures (methane varying from 1 to 10 %) are studied, with a monitoring 15
of the methane consumption and of the stable gas neutrals by in-situ mass spectrometry. 16
Atomic hydrogen is also measured by optical emission spectroscopy. A positive correlation is 17
found between atomic hydrogen abundance and the inhibition function for aerosol production. 18
This confirms the suspected role of hydrogen as an inhibitor of heterogeneous organic growth 19
processes, as found in [Sciamma-O’Brien et al. (2010) Icarus, 209, 704]. The study of the gas 20
phase organic products is focussed on its evolution with the initial methane amount [CH4]0 21
and its comparison with the aerosol production efficiency. We identify a change in the 22
stationary gas phase composition for intermediate methane amounts: below [CH4]0=5%, the 23
gas phase composition is mainly dominated by Nitrogen-containing species, whereas 24
2
hydrocarbons are massively produced for [CH4]0>5%. This predominance of N-containing 25
species at lower initial methane amount, compared with the maximum gas-to solid conversion 26
observed in Sciamma-O’Brien et al. 2010 for identical methane amounts confirms the central 27
role played by N-containing gas-phase compounds to produce tholins. Moreover, two 28
protonated imines (methanimine CH2=NH and ethanamine CH3CH2=NH) are detected in the 29
ion composition in agreement with Titan’s INMS measurements, and reinforcing the 30
suspected role of these chemical species on aerosol production. 31
32
3
1-Introduction 33
On Titan, the dissociation of N2 and CH4 by solar UV radiation, cosmic rays and Saturn’s 34
magnetosphere electron bombardment induces a complex organic chemistry at high altitudes 35
(Waite Jr. et al., 2007) that results in the production of solid aerosols responsible for the 36
orange haze surrounding Titan. These are the most complex extraterrestrial organic material 37
detected in the solar system. Their chemical production mechanisms are roughly described, 38
first by a photolytic priming of the nitrogen and methane precursors, then by the formation of 39
hydrocarbon and nitrile monomers in the gaseous phase, finally followed by polymerization 40
processes leading after recombination to solid particles big enough to condensate, 41
agglomerate and settle down to the ground. However, if one looks in the detail, each step 42
identified in the chemical growth process is subject to research and to continuous 43
developments. The work of Imanaka et Smith, 2007 showed the central role of N2 ionization 44
for gas products formation. Pernot et al., 2010 suggested new routes towards polymerization, 45
complementary to the previous acetylene, benzene, HCN and HC3N routes implemented in 46
Titan's photochemical models (Lebonnois, 2005; Lebonnois et al., 2002). 47
Several experimental setups have been developed in order to reproduce and study in the 48
laboratory such a complex atmospheric system. Among them, the plasma device PAMPRE 49
provided significant clues on the understanding of the polymeric chemical structure of the 50
aerosols (Carrasco et al., 2009; Pernot et al., 2010). The influence of the methane initial 51
concentration on the aerosol mass production efficiency was moreover studied in Sciamma-52
O'Brien et al., 2010. A maximum has been found for the intermediate initial concentrations of 53
methane. This important result highlights a competition between a polymerization growth 54
process correlated with the methane concentration, and an inhibition process anti-correlated 55
with the methane concentration. Several hypotheses were proposed to explain this inhibition 56
process in the aerosol production, all of them involving the increase of the global hydrogen 57
4
content in the gas phase coming from methane dissociation: either by saturation of the 58
growing solid grains with hydrogen (molecular or atomic), or by production in the gas phase 59
of saturated volatiles less favourable to achieve polymerization. In order to find some clues on 60
this sensitivity of the aerosol production with the methane initial content in the gas mixture, 61
we began to study the evolution of the gas phase products composition according to this 62
initial methane amount. The stable neutrals were detected by in-situ mass spectrometry. 63
Moreover a relative quantification of the atomic hydrogen content was performed by in-situ 64
Optical Emission Spectroscopy (OES). All the in-situ products found in this work are to be 65
compared with the parallel study of Gautier et al., 2011 on the same PAMPRE reactive gas 66
mixtures analysed by cryogenic trapping and Gas Chromatography-Mass Spectrometry (GC-67
MS) and with the previous gas mixture analysis on similar plasma experiments (Coll et al., 68
1999; De Vanssay E et Raulin, 1995; Ramírez et al., 2001; Ramírez et al., 2005). 69
70
5
2-Experimental 71
2.1. The PAMPRE plasma reactor 72
The PAMPRE experimental setup has been described in detail in previous publications 73
(Alcouffe et al., 2010; Szopa et al., 2006). Briefly, the reactor consists of a cylindrical 74
stainless steel chamber. Two gas bottles, one of pure N2 and one containing a N2-CH4 mixture 75
at 10% CH4 concentration, are used to obtain different gas mixtures in the experiment. A third 76
bottle of Argon is used to introduce Argon as an actinometer (see Part 2.3.). The Radio 77
Frequency Capacitively Coupled Plasma (RF-CCP) discharge is driven by a 13.56 MHz 78
frequency generator with power up to 100 W, and generated between two electrodes: one 79
grounded, one polarized. The discharge is confined by the grounded electrode which is a 80
cylindrical grid box, 13.7 cm in diameter and 4 to 5 cm in height. The N2-CH4 gas mixture is 81
continuously injected through the polarized electrode and pumped by a rotary vane vacuum 82
pump. With this discharge design, the gas flow is uniform in the confining cylindrical box. 83
In the study presented here, the RF power is fixed at 30 W, the total N2-CH4 gas flow rate at 84
55 sccm and the gas pressure at 0.9 mbar for all the experiments. A constant argon gas flow 85
rate of 2.75 sccm (corresponding to an amount of ~5%) is added to the gas mixture for 86
actinometry measurements. This compound does not participate to the gas phase chemistry, 87
and its low amount does not modify the plasma discharge. 88
The low pressure RF discharge produces a “cold plasma”, which is not at thermodynamical 89
equilibrium. The neutral gas remains at ambient temperature (Alcouffe et al., 2010), when 90
electron have an mean energy on the order of one or two eV. The assumed maxwellian 91
electron energy distribution mimics quite correctly the photon solar energy distribution 92
(Szopa et al., 2006). Neutrals are warmer in the PAMPRE experiment than in Titan’s 93
6
atmospheric conditions, globally enhancing the kinetics of neutral-neutral bimolecular 94
reactions. 95
Before each experiment, the reactor chamber, plasma box and crystallizer are cleaned with 96
ethanol. The reactor is then pumped down to secondary vacuum by a turbo molecular pump 97
down to 2×10-6 mbar, and baked out at 110°C for several hours in order to remove impurities 98
adsorbed onto the chamber walls. Once the chamber has cooled down, a 30 minutes argon 99
plasma ensures the outgassing of the metallic confining cylindrical grid box. 100
2.2. Mass spectrometry 101
In situ measurements of the gas phase composition are achieved using a Pfeiffer QME 200 102
quadrupole mass spectrometer (MS). In the MS, the neutral molecules are ionized by electron 103
impact at the standard 70 eV electron energy. The gas sampling is done through a capillary 104
tube (0.8 mm in diameter), with a sampling orifice located 1 cm outside the reactive plasma. 105
The capillary is long enough to reduce the gas flow rate between the reactor chamber and the 106
MS and keep the operating pressure in the MS below 10-5 mbar. As a consequence, only 107
stable molecules can be measured with this experimental set up. The MS detector has a 108
resolution of 1 u and covers the 1 to 100 u range. 109
A weak air signature (N2, O2, CO2) is highlighted in the mass spectra leading to contributions 110
at m/z 14, 16, 28, 32 and 44, even in the blank of the mass spectrometer given Figure 1. The 111
blank corresponds to the isolated mass spectrometer functioning at a vacuum limit of about 112
3×10-8 mbar. The water signal at m/z= 2, 16, 17, 18 is quite important, reaching an intensity 113
of about 1×10-10 A for m/z=18 in all the spectra including the blank and remained stable 114
during the experiments. This signal is due to remaining water adsorbed on the wall of the 115
QMS housing because a very high temperature outgassing was not possible with the used 116
configuration. Figure 1 also shows a spectrum recorded for a 5% methane concentration in the 117
PAMPRE reactor when the plasma is OFF. The m/z = 18 signal remains the same. An 118
7
intensity threshold for recording has been chosen equal to 10-13 A. Studying the stationary 119
state allows long acquisition time-scales: a 2 seconds shift is chosen for each mass between 120
m/z 2 to 80. 121
122
1.00E-14
1.00E-13
1.00E-12
1.00E-11
1.00E-10
1.00E-09
1.00E-08
0 5 10 15 20 25 30 35 40 45 50 55 60
m/z
Inte
nsity
(A
)
123
Figure 1: (Black) Blank mass spectrum of the mass spectrometer only. The low residual air signal is 124
consistent with the vacuum limit pressure of 3×10-8 mbar. The quite important water signal, at m/z=18, 17, 125
16 and 2 is stable, and corresponds to residual water adsorbed in the QMS. (Red) Initial mass spectrum 126
obtained with a N2-CH4 gas mixture of [CH4]0=5%. 127
128
The methods used are systematically both scan analog (range of masses continuously scanned 129
and recorded) and scan bargraph mode (only intensities of unit mass peaks are recorded). The 130
bargraph mode is faster and allows a direct comparison with the NIST database (Stein, 2011), 131
but we observed that this acquisition mode can miss some products with mass peak in the tail 132
of an adjacent predominant species. An example is given on Figure 2 showing the respective 133
analog and bargraph acquisitions for the same gas mixture obtained after a plasma discharge 134
8
in a 1% methane gas mixture. The HCN production (m/z 27) is visible in the analog scan, but 135
has been completely missed in the bargraph mode because of the major N2+ fragment (m/z 136
28). Both acquisitions were thus systematically recorded and cross-compared, helping to 137
correct the possible incomplete bargraphs. 138
139
1,0E-14
1,0E-13
1,0E-12
1,0E-11
1,0E-10
1,0E-09
1,0E-08
0 5 10 15 20 25 30 35 40 45 50 55 60
m/z
Inte
nsity
(A
)
140
Figure 2: Comparison between a scan analog (red line) and a scan bargraph acquisition (black squares) 141
on the example of the stationary state plasma discharge in a 1% methane initial gas mixture. 142
143
2.3. Optical emission spectroscopy and actinometry 144
The concentration of atomic hydrogen cannot be studied with our mass spectrometry device, 145
which measures only stable neutrals. Optical Emission Spectroscopy (OES) is a non-intrusive 146
diagnostic method commonly used for in situ studies of plasma discharges. In our conditions, 147
the H atoms produced from CH4 dissociation are excited by electronic collision to radiative 148
levels. These levels are then depopulated by radiative emission. The intensity of an H line is 149
given by: 150
9
( )[ ]HTknHA
HAHhCVHI eHee
ji
jiHO −
∑=
)(
)()()()(
,
,νλ (1) 151
• where VO is the volume of the plasma observed by OES, 152
• C(λH) is a spectral calibration factor of the optical device for the chosen hydrogen line 153
wavelength, 154
• hν(H) is the photon energy, 155
• A i,j the Einstein coefficient for the observed line, 156
• ∑ )(, HA ji =1/τ (τ is the radiative life time of the level). 157
These five terms are independent on the plasma and on the H concentration. 158
The three last are plasma depending: ne is the electron density, ke-H(Te) the rate coefficient for 159
the population of the radiative level, function of the electron temperature Te and [H] is the 160
atomic hydrogen density. 161
Equation (1) shows that even if an H line intensity is proportional to [H], it depends also on 162
plasma parameters. 163
In order to avoid this problem, we add into the plasma a small amount of argon as 164
actinometer. Argon is non-reactive and, in our experimental conditions, its radiative levels are 165
populated only by electron collisions and depopulated by spontaneous emission as H lines. 166
Then, an Ar line intensity is given by: 167
( )[ ]ArTknArA
ArAArhCVArI eAree
ji
jiArO −
∑=
)(
)()()()(
,
,νλ (2) 168
where VO is the same observed volume as for the H line and C(λAr) is the spectral calibration 169
factor at the Ar line wavelength. The Ai,j are the Einstein coefficients for the Ar line. 170
Combining equations (1) and (2) the H atoms density is given by: 171
[ ] ( )( )
( )( ) [ ]ArTk
Tk
ArI
HIAH
eHe
eAre
−
−= (3) 172
10
As mentioned before, the Ar amount is constant. So, equation (3) shows that [H] is 173
proportional to the ratio of the intensity of select H and Ar lines and not to electron density: it 174
is the optical actinometry method. The question of the electronic rate coefficients will be 175
discussed below. 176
The A factor depends only on spectroscopic data of selected lines and spectral calibration, but 177
not on the observed volume. 178
Before using optical actinometry, we have to check if the addition of Ar in the gas mixture 179
modifies the plasma discharge. As explained in Alcouffe et al. (2010) the self-bias voltage Vdc 180
of the polarized electrode is directly correlated with electron density and temperature. No 181
variation of Vdc has been measured when argon is added to the discharge. 182
For OES, the light emitted by the plasma is dispersed by a 60 cm focal length monochromator 183
(Jobin-Yvon) connected to a photomultiplier (Hamamatsu R928). The output current of the 184
photomultiplier is measured with a picoammeter (Keithley 6485) connected to a computer 185
(Alcouffe et al., 2010). Spectra are recorded in the visible to the near IR using a colored glass 186
filter to eliminate overlapping of second order lines in the red and near IR range. 187
A spectrum of the discharge from 475 nm to 825 nm is presented in Figure 3. The spectrum is 188
clearly dominated by nitrogen molecular bands emission of second positive system (SPS) 189
N2(C→ B) and first positive system (FPS) N2(B→A), hydrogen emission of Hβ (486.1 nm) 190
and Hα (656.3 nm), and atomic Ar lines . 191
192
11
475 480 485 490
0
10
20
30
640 645 650 655 6600
50
100
150
740 745 750 755 7600
200
400
600
810 815 820 8250
50
100
150
*
N2(B
,A)(
9,6)
N2(B
,A)(
8,5)
Ar,
826
.9nm
Ar,
811
.5nm
Ar,
810
.4nm
N2(B
,A)(
7,4)
Ar,
751
.5nm
Ar,
750
.4nm
H ββ ββ
N2(C
,B)(
2,8)
Inte
nsity
, a.u H αα αα
5 % CH4/ 5 % Ar/ N
2
Wavelength, nm 193
Figure 3: Typical example of emission spectrum obtained between 475 and 828 nm for 5% CH4/N2 RF 194
plasma with the presence of 5% Ar as an actinometer gas. The mean features emissions are identified 195
together their wavelengths. The asterisk points out the N2(B, A)(6, 3) emission line. 196
197
Gicquel et al., 1998 have used optical actinometry with Ar to determine H concentration in 198
H2-CH4 plasma. In order to have limited variations of the electronic rate constants in the 199
equation (3), Gicquel et al. use the Hα (656.3 nm) and the 750.3 nm Ar lines whose excitation 200
energies are on the same order of magnitude (respectively 12.1 eV and 13.5 eV), and the cross 201
sections for excitation by electron collisions are similar. 202
But in our case, Figure 3 shows that in N2-CH4 plasma the Ar (750.4 nm) line is 203
superimposed to the head band of the (4-2) and the Hα line is embedded in the rotational 204
spectra of the (6-3) band of the FPS. On the opposite, the Hβ (486.1 nm) as Ar (811.5 nm) 205
lines are well isolated. As their excitation energies, 12.74 eV for Hβ and 13.07 eV for Ar 206
12
(811.5 nm), are on the same order of magnitude and in agreement with the criteria defined by 207
Gicquel et al., 1998, these two lines are chosen for optical actinometry measurements. 208
209
13
3-Results and discussion 210
Various initial gas mixtures are studied, with 1, 2, 5, 8 and 10% of methane in nitrogen, 211
covering the methane concentration range of our previous study on the gas-to solid 212
conversion yield (Sciamma-O'Brien et al., 2010). 213
3.1. Atomic hydrogen 214
Optical Emission Spectroscopy measurements have been done for plasma conditions with 215
initial amount of CH4 from 1 to 10% in nitrogen. The argon amount was always the same 216
(about 5%). Figure 4 shows the intensity of Hβ (486.1 nm) and Ar (811.5 nm) lines as a 217
function of the CH4 percentage. 218
0 2 4 6 8 10
50
100
150
200
b: Ar(811.5nm)
Inte
nsity
, a.u
% CH4
0 2 4 6 8 103
6
9
12
Inte
nsity
, a.u
a: Hββββ(486.5nm)
219
Figure 4: Dependence of (a) Hβ (486.1 nm) and (b) Ar (811.5 nm) emission line intensities on the 220
percentage of CH4 added to N2 RF plasma. The percentage of Ar as an actinometer gas is 5% for all 221
measurements. 222
223
14
The Hβ emission reaches a maximum for about 5% of CH4. Whereas the argon density is 224
constant, Ar line varies with the CH4 amount. As shown in equation (2), this variation is 225
related to the electron density and temperature. In order to remove the influence of electron 226
density, the ratio Hβ/Ar is calculated and presented in Figure 5. 227
Equation (3) shows that this ratio is proportional to H density, [H]. 228
229
Figure 5: Evolution of hydrogen atom densities as a function of % CH 4 obtained from Figure 4 in the 230
same experimental conditions. 231
232
From Figure 5, it can be deduced that [H] increases linearly with the injected amount of CH4. 233
The linearity is valid only if the ratio ke-Ar(Te)/ke -H(Te) remains constant. Alcouffe et al., 2010 234
have shown that the electron temperature varies when solid particles are present in the plasma. 235
Gicquel et al., 1998 have calculated that this ratio variation is only about 10% when electron 236
temperature changes from 1eV to 5eV. As in our experimental conditions electron energy 237
15
remains on the order of few eV, we can consider that the ratio is constant and thus that [H] 238
increases linearly with the injected amount of CH4. 239
This positive linear increase with [CH4]0 is anti-correlated with the decrease of the gas-to-240
solid conversion efficiency measured in Sciamma-O'Brien et al., 2010. As a consequence, 241
atomic hydrogen provides a possible important clue to understand inhibition of the aerosol 242
production observed when the amount of methane increases. 243
244
3.2. Stable neutrals. 245
Before studying the reactive plasma, some general comments can be made on the reference 246
gas mixture mass spectra before plasma ignition (see Figure 6). The initial mixture is mainly 247
composed by molecular nitrogen and methane, which can be identified on the mass spectrum 248
with their following fragments. Molecular nitrogen involves fragments at m/z = 7 (N2+), 14 249
(N+), 15 (15N+) 28 (N2+), 29 (15N14N+) and 30 (15N15N+). Methane exhibits fragments at m/z 250
=12, 13, 14, 15 and 16, respectively C+, CH+, CH2+, CH3
+ and CH4+. Additional argon is 251
introduced for complementary optical emission spectroscopy measurements, leading to large 252
contributions at m/z = 20, 36 and 40. 253
254
16
0 5 10 15 20 25 30 35 40 45 50 55 6010-14
10-13
10-12
10-11
10-10
10-9
10-8
Inte
nsity
, a.u
m/z
Plasma OFF Plasma ON
255
Figure 6: Mass spectrum of a CH4-N2 10-90% gas mixture, (black) plasma OFF, (red) plasma ON. 256
257
As the absolute intensity measured by the mass spectrometer is not faithfully reproducible, 258
mainly due to the progressive clogging of the filament in the ionization chamber and a slow 259
drift of the amplification factor of the channeltron, we chose to work with relative intensities 260
among the products. For each experiment series, mass spectra are recorded before the plasma 261
discharge. The initial state is hereafter called initial condition and labelled with the subscript 262
0. When the plasma is on, mass spectra are recorded after the transient regime when the 263
discharge is at the stationary state (Sciamma-O'Brien et al., 2010). This stationary state is 264
labelled with the subscript SS. The spectra are therefore normalized with a peculiar correction 265
factor [N2]0/I0(28) where I0(28) is the intensity current (A) of the mass peak m/z 28 and [N2]0 266
the nitrogen concentrations in the initial gas mixture (plasma off). When normalized, spectra 267
are expressed in arbitrary units (AU). 268
17
3.2.1. Molecular Hydrogen 269
Information of molecular hydrogen production is contained in the m/z = 2 intensity 270
production Iss(2)-I0(2), difference between stationary state and initial mass spectra. Indeed this 271
indicator informs us about the global hydrogen content in the reactive plasma, summing the 272
major contribution of molecular hydrogen, but also of atomic hydrogen efficiently 273
recombined on the metallic sampling line before the ionization chamber of the mass 274
spectrometer. 275
Secondary contributions from hydrocarbon products fragmentation are negligible. Indeed, as 276
shown on Figure 6, the m/z=2 signal increases from 8×10-11A to 1×10-9A. A similar 277
production of a few 1×10-9A can be observed for the peak at m/z=28. It corresponds to the 278
ethane C2H4 signature, which has no significant contribution at m/z=2 in its fragmentation 279
pattern (NIST and Pfeiffer databases). All the other products peaks are at the most of the 280
order of 1×10-11A. The fragments of these products can therefore not contribute significantly 281
to the m/z=2 signal. 282
The contribution of atomic hydrogen can also be considered as negligible if we compare with 283
the concentrations of H2 and H predicted by Pintassilgo et Loureiro, 2010 for a N2-CH4 284
afterglow plasma model, corresponding to ten times less H atoms than H2 molecules. As a 285
consequence, Iss(2)-I0(2) can at first order be assimilated to molecular hydrogen signature. 286
Figure 7 reports the molecular Hydrogen production (difference between the stationary Iss(2) 287
and the initial I0(2) intensities of the mass peak at m/z=2), as a function on the initial methane 288
concentration (upper panel), or as a function of the consumed methane (lower panel). Error 289
bars are calculated as ±2σ on the statistical among several recorded spectra for each condition. 290
The uncertainty is larger for the [CH4]0=8% conditions because of an unusual drift of the 291
signal during this specific experiment. 292
293
18
0 2 4 6 8 100.0
0.1
0.2
0.3
0.4
0.5
I ss
(2)-
I 0(2)
(AU
)
[CH4]0 (%)
-0.05 -0.04 -0.03 -0.02 -0.010.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
I SS
(2)-
I 0(2
) (A
U)
ISS
(15)-I0(15) (AU)
294
Figure 7: Hydrogen production (difference between the stationary Iss(2) and the initial I0(2) intensities of 295
the mass peak at m/z=2) (above) according to the initial methane concentration in the gas mixture; (below) 296
according to the methane consumption (difference between the stationary Iss(15) and the initial I0(15) 297
intensities of the mass peak at m/z=15). The methane fragment CH3+ at m/z = 15 has been chosen because 298
it does not overlap with water fragments. 299
19
We observe on Figure 7 (upper panel) that Iss(2)-I0(2) increases globally with the initial 300
methane content, but reaches an asymptote for initial concentrations higher than 5% of 301
methane. This can be related to the same asymptote observed in Sciamma-O'Brien et al., 2010 302
for methane consumption Iss(15)-I0(15). Both parameters are strongly correlated as can be 303
seen on the lower panel of Figure 7. This H2 enrichment with the initial concentration of 304
methane is consistent with the time-dependent measurement carried out by Majumdar et al., 305
2005 with a 2:1 CH4-N2 gas mixture. Moreover, this result shows that molecular hydrogen 306
concentration is dominated by the methane dissociation processes more than by all the 307
secondary reactions producing H2 and occurring in the plasma (neutral-neutral, ion-molecule 308
and ion dissociative recombination reactions). 309
On the contrary, H concentration increases linearly with initial methane concentrations 310
(Figure 5). No asymptote is observed for initial methane concentrations higher than 5% as for 311
molecular hydrogen. This suggests that atomic Hydrogen production is more independent 312
from methane direct dissociation than molecular Hydrogen. Atomic Hydrogen is probably 313
more linked with other reactions as for example dissociative recombination of positive ions 314
(Plessis et al., 2010). 315
316
3.2.2. Ammonia 317
Ammonia NH3 is of great interest concerning prebiotic chemistry, providing reactive 318
nitrogen-containing species to Titan’s atmosphere. It has been detected and quantified by the 319
INMS instrument onboard Cassini and are suggested to be chemically correlated (Yelle et al., 320
2010). 321
In our case, the ammonia production signature at m/z=17 is partially hidden by the 322
contribution of the water signal of the mass spectrometer. In order to extract this information 323
from the data, we precisely quantified the I17/I18 ratio given by the only water signature in our 324
20
QMS, using all the spectra recorded without plasma, so in absence of ammonia. Thirty eight 325
spectra have been recorded and a statistical fluctuation of I17/I18 can be deduced. A maximum 326
error bar of ±2σ is calculated and plotted for each experimental condition in Figure 8. The 327
I17/I18 measurement outside this statistical water range corresponds thus to a significant 328
difference compared with the water ratio. A larger value of I17/I18 is consequently significant 329
for the production of ammonia when the plasma is on and contributing to I17. 330
An increase of the ammonia production with the initial amount of methane can actually be 331
observed on Figure 8, and becomes significant for methane concentrations higher than 5%. 332
An ammonia production is not discarded for methane concentration below 5% but it is not 333
important enough to be distinguished from the experimental uncertainty of the I17/I18 water 334
ratio. 335
336
21
337
0 2 4 6 8 10
0,44
0,48
0,52
0,56
0,60
Plasma ON Plasma OFF
I 17 /
I 18
[CH4]0 (%) 338
Figure 8: Intensity ratios I17/I18 as a function of the initial methane concentration (red) when the plasma is 339
ON, (black) when the plasma is OFF. The uncertainties calculated corresponds to the 2σσσσ statistical 340
experimental scattering, calculated with 3 to 8 experiments when the plasma is ON for each methane 341
percentage, and with 30 spectra when the plasma is OFF (statistics on the whole set of experiments, 342
whatever the initial methane percentage). 343
344
Ammonia formation in CH4-N2 plasmas is an important issue which has not been definitely 345
solved yet (Horvath et al., 2011; Mutsukura, 2001; Uyama et Matsumoto, 1989). In any case 346
the key is the production of NH radicals, which react with molecular hydrogen to produce 347
ammonia through the reaction 348
NH + H2 � NH3 (4) 349
This reaction explains that ammonia production is promoted by a hydrogen-rich gas mixture. 350
The NH radical is suggested in the literature to be both produced by gas phase mechanism, 351
involving ion chemistry: 352
22
N2++CH4 � N2H
+ + CH3 (5) 353
N2H++e-
�NH+N (6) 354
and by radical chemistry: 355
N+ H�NH (7) 356
NH+ H2�NH3 (8) 357
Some studies suggests that the radical pathway can in some conditions be catalysed on the 358
surface of the reactor (Touvelle et al., 1987). However, in our experimental device, we can 359
assume that reactions are mainly in the plasma volume and that this wall effect can be 360
neglected. The ratio volume/reactor surface is actually larger in our case than in the set up of 361
Touvelle et al., 1987. 362
In addition to the identified and well-known mechanisms listed above, dissociative 363
recombination with electron of protonated amines may also provide plausible pathways in the 364
plasma towards ammonia, as the one suggested by Yelle et al., 2010 for Titan’s atmospheric 365
chemistry from protonated methanimine dissociative recombination (eq. 11). 366
We are in need of further experimental data on the dissociative recombination of nitrogeneous 367
ion species to evaluate this possible ammonia source in both Titan’s ionosphere and our 368
plasma experiment. 369
370
371
23
3.2.3. Organic species production 372
Figure 6 illustrates the rich product formation in the stationary plasma for an initial methane 373
concentration of 10%: a production of species can be observed in the ranges m/z=[24-29], m/z 374
= [36-44] and m/z = [50-55]. These mass ranges correspond respectively to elemental formula 375
of CxHyNz, x+z=2, CxHyNz, x+z=3,. CxHyNz, x+z=4. Those will be named C2, C3 and C4 376
blocks in the following. Ionization of a complex mixture of neutrals by electronic impacts 377
with electrons at energy of 70 eV leads to overlapping cracking patterns in the mass spectra, 378
which are commonly reported as difficult to interpret and set up a recurring issue for spectra 379
analysis. Only the main univocal mass peaks are often treated (Deschenaux et al., 1999; 380
Majumdar et al., 2005). 381
382
3.2.3.1. General evolution with methane initial amount 383
For this reason, we first analysed the evolution of the mass intensities according to the 384
methane amount, without any temptative identification. Results are plotted in a semi log scale 385
on Figure 9:, with 2σ uncertainties on the measurements calculated by a four times repetition 386
of each spectrum measurement. 387
388
24
0 2 4 6 8 101E-4
1E-3
0,01
0,1
Inte
nsity
(A
U)
[CH4]0 (%)
28
27
26
29
25
C2-block
0 2 4 6 8 10
1E-5
1E-4
1E-3
C3-block
38
44
37
Inte
nsity
(A
U)
[CH4]0 (%)
41
43
0 2 4 6 8 101E-5
1E-4
1E-3
C4-block
Inte
nsity
(A
U)
[CH4]0 (%)
5251
54
5053
Figure 9: Evolution of significant peaks detected, according to the initial methane amount. Reported 389
uncertainties are 2σ calculated on four repetitions of each spectrum and two experiments per methane 390
condition. 391
25
392
Globally, we observe an important effect of the initial amount of methane in the gas mixture, 393
with more and more gaseous species, also with larger intensities, detected when the initial 394
methane concentration increases. Less products peaks are observed in the mass spectrum for 395
the 1% methane condition, in comparison with the 10% methane one. This result is in 396
agreement with the previous study of Sciamma-O’Brien et al. (2010) on the gas-to-solid 397
conversion efficiency. It was shown that this efficiency decreases drastically with the initial 398
methane concentration, meaning that for similar methane consumption, the gas mixture 399
becomes more and more favourable to gaseous products in the detriment of solid products. 400
401
3.2.3.2. Identification of the species 402
Differential mass spectra for all the detected peaks (ISS(X)-I0(X)) have been measured for 403
[CH4]0 = 1 to 10%. Results obtained at 1, 5 and 10 % of methane in the initial gas mixture are 404
plotted in Figure 10. 405
406
26
407
20 25 30 35 40 45 50 55 60-6
-5
-4
-3
-2
-1
0
Log(
I SS- I 0)
m/z
1% CH4
5% CH4
10% CH4
408
Figure 10: Differential mass spectra (Stationary spectrum – Initial spectrum) for initial gas mixtures of 409
(black) 1%, (blue) 5% and (red) 10% of methane in the initial gas mixture. 410
411
Differential mass spectra for [CH4]0=1%: explicit treatment. 412
We observe that for this initial methane concentration, fewer molecules are produced in the 413
plasma. The identification is less ambiguous and the spectra can be treated in details. Mass 414
peaks can be singly attributed as follows. 415
The heaviest detected block, of C4 compounds is dominated by the peak at m/z = 52 416
and is attributed to C2N2, which has only one other fragment in the NIST database at m/z = 26 417
(5% of the m/z = 52 intensity). C4H4 is not possible because its fragments at 50 and 51 amu, 418
which are important in the NIST database (50% of the main m/z = 52 peak) are in 419
disagreement with the observed mass spectrum. C2N2 comes from the recombination of two 420
CN radicals. C2N2 has previously been observed in Titan's atmosphere (Cui et al., 2009; 421
27
Robertson et al., 2009; Vuitton et al., 2007) and in N2-CH4 plasma experiments (Bernard et 422
al., 2003; Majumdar et al., 2005; Ramírez et al., 2005). CN radicals have moreover been 423
detected by optical emission spectroscopy in N2-CH4 plasmas (Alcouffe et al., 2010; Horvath 424
et al., 2011). 425
The C3 block includes 4 main peaks at mass m/z 44, 43, 41 and 38. 426
Propane (C3H8) is the most probable product at m/z 44. The odd masses at m/z = 41 and 43 427
can be attributed to one nitrogen-bearing species. Acetonitrile (CH3CN) is unambiguously at 428
m/z = 41, and has been cross-confirmed as a major product in the experiment by the GCMS 429
analysis of Gautier et al., 2011. The mass peak at m/z = 43 is the signature of ethanimine 430
CH3-CH=NH (and/or amino-ethene with the unsaturation carried by the C=C bond: CH2=CH-431
NH2), which is cross-confirmed by the ion detection of Mutsukura, 2001 (see section 3.3). 432
Ethanimine has been shown to be produced by the reaction between N(2D) and C2H4 433
(Balucani et al., 2010). However, it is not to exclude that this mass peak may also be the result 434
of a fragmentation pattern of amino-methanimine (H2N-CH=NH), its fragmentation pattern 435
being unknown. However, the ion study of Mutsukura, 2001 does not show the signature of 436
amino-methanimine. Further studies with complementary in situ measurements could be 437
necessary to unveil the ambiguity. 438
The m/z 38 mass peak is observed in all the reactive N2-CH4 studies made by mass 439
spectrometry (Imanaka et Smith, 2010; Majumdar et al., 2005), but there is not clear evidence 440
to attribute this fragment yet. A C2N+ fragment from the C2N2 neutral can be suspected. 441
442
The C2 block shows significant peak intensities for m/z = 25, 26, 27, 28 and 29. They are 443
in agreement with the similar N2-CH4 plasma studies performed by Majumdar et al., 2005. 444
Three main points can be extracted from these C2 block data, largely complicated by the 445
overlapping of several hydrocarbon and nitrogenous containing fragments. 446
28
• The HCN formation (main peak at m/z = 27), is cross identified as the major 447
gaseous products by Gautier et al., 2011 in the trap analysis. 448
• An important intensity of the m/z = 28 peak, corresponds mainly to ethylene C2H4. 449
Indeed, as we analyse the differential spectrum between final and initial states, the 450
increase of the m/z 28 peak cannot be due to a production of N2. 451
• The production of acetylene C2H2 at m/z = 26 is possible but its main fragment at 452
m/z = 26 is mixed with the contributions of the CN+ fragments of all abundant 453
nitriles. 454
A firm detection of methanimine, CH2=NH at m/z = 29 can be given for [CH4]0=1%. Actually 455
an additional contribution at m/z = 29 can be given by the propane fragmentation 44(50) 456
29(100) pattern (database of the Pfeiffer QMS 200). The ratio of these two peaks in the 457
differential spectra is therefore plotted Figure 11 as a function of initial percentage of 458
methane. For [CH4]0 larger than 2%, the ratio is constant and consistent with the propane 459
pattern, showing that this compound becomes the main contributor to the m/z = 29 peak for 460
these methane amounts. On the other hand, the ratio is much larger for [CH4]0=1%, and 461
becomes inconsistent with propane. It confirms the important contribution of methanimine at 462
m/z = 29 in these experimental conditions. Methanimine was previously detected in a N2-CH4 463
microwave plasma discharge by Fujii et Arai, 1999; Fujii et Kareev, 2001; Kareev et al., 464
2000. However, micro-wave discharges are well-known to be very warm and to be compared 465
with combustion processes. Our study confirms thus the methanimine production in much 466
cooler experimental conditions, more representative of the cold Titan’s atmospheric 467
chemistry. Methanimine has been detected and quantified by the INMS instrument onboard 468
Cassini (Yelle et al., 2010) and is mainly produced by the reaction between N(2D) and 469
methane (Balucani et al., 2009). 470
29
0 2 4 6 8 100
2
4
6
8
10
[Iss
(29)
- I 0(2
9)] /
[Iss
(44)
- I 0(4
4)
[CH4]0, %
471
Figure 11: Evolution of the ratio between the increases of m/z 44 and m/z 29 when the plasma is ON. Two 472
experiments were performed per [CH4]0 conditions. The stable value of about 2 for [CH4]0 concentration 473
larger than 2% is in agreement with a co-evolution of fragments intensities for propane. However, the 474
larger value of this ratio for [CH 4]0=1% indicates an additional contribution of methanimine at mass 29 u. 475
476
Table 1 summarizes the identified molecules from in the differential mass spectra at 477
[CH4]0=1%. 478
C2 block
m/z 26 27 28 29
Molecule
HCN
(CN
fragment)
HCN C2H4 CH2=NH
C3 block m/z 38 41 43 44
Molecule C2N2 ? CH3CN CH3-CH=NH C3H8
C4 block m/z 52 - - -
Molecule C2N2 - - -
Table 1: Molecules identified in the differential mass spectra at [CH4]0=1%. 479
30
Gas mixture with [CH4]0 > 1 %: Single Value Decomposition. 480
The spectra obtained at higher amount of methane in nitrogen are more complex than in the 481
1% case. Single-Value Decomposition (SVD) algorithms have been used for the treatment of 482
the Cassini INMS neutrals mass spectra (Cui et al., 2009) and of a photochemical 483
experimental simulation of Titan's atmosphere (Imanaka et Smith, 2007). These systematic 484
methods improve notably the processing of the spectra, but their non unique decomposition 485
results are also known to be very dependent on the chosen database and to be taken with a lot 486
of care. Studies on quantification and identification were performed by Robert, 2010; Turner 487
et al., 2004. They showed that QMS instruments with electron impact ionization methods 488
have strong non-linearities, and cross-sensitivity effects (dependence of the response of a 489
compound according to its co-mixture with other species). A proper calibration is almost 490
impossible for gas mixture more complex than 5 species. In our case, the GC-MS study of 491
Gautier et al., 2011, has revealed a gas mixture of about 40 different species, often non-492
commercially available. We therefore made use of such decomposition tools in order to go 493
further in the understanding of the gas mixture composition, but we have been very careful 494
not to over-interpret the data inversion. 495
Our reference database was built according to the following rules. We used preferentially the 496
specific Pfeiffer database provided with the instrument, which is significantly different from 497
the NIST (Stein, 2011) fragmentation patterns for ionization by a 70 eV electronic impact. We 498
found for N2 and CH4 some differences between the NIST and the Pfeiffer database of about 499
20-30% on the fragmentation patterns. For species absent from the Pfeiffer database but 500
present in the NIST, the reference mass spectra were taken from the NIST. 501
The SVD decomposition was limited on the mass range above m/z 20, for CxHyNz species 502
with x+z ≥ 2, adapted to products identification. As in Imanaka et Smith, 2010, species were 503
added progressively and a minima. 504
31
A few clear conclusions could however be obtained. 505
• The main peaks of the spectrum can be easily fitted with nitriles: HCN (27 u), CH3CN 506
(41 u), C2H3CN (53 u), C2H5CN (55 u), HC3N (51 u) and C2N2 (52 u) and are 507
consistent with the gas-phase studies of Coll et al., 1999; De Vanssay E et Raulin, 508
1995; Gautier et al., 2011; Ramírez et al., 2001; Ramírez et al., 2005. 509
• Hydrocarbons have complex fragmentation patterns and do not contribute explicitly to 510
specific and univocal mass peaks. We were therefore only able to confirm that their 511
introduction is useful to improve the fit, but they are not explicitly constrained by the 512
mass spectrum and cannot be strictly identified. As a consequence, we suspect that the 513
attribution made in the comparable study of Imanaka et Smith, 2010, provides also 514
estimations but cannot be considered as a firm detection. 515
• Moreover, no hydrocarbon allows to correctly simulate the peaks m/z = 54, 38, 37 and 516
36. Because of the fragmentation pattern description weaknesses, we cannot conclude 517
that a species is missing to explain masses 36, 37 and 38 u. Indeed they are located on 518
the lower masses of the C3 block. They also could be due to the fragments of larger 519
hydrocarbons, poorly taken into account. However, it is not the case for the mass 54 520
which is located on the right side of the C4 block. The impossibility to fit the mass 521
spectrum with well-known hydrocarbons suggests a nitrogen bearing species, not yet 522
taken into account in the database. The condensation of HCN and methanimine 523
(H2C=N-CN or HN=CH-CN) could possibly explain such an m/z 54 peak. 524
525
3.2.3.3. Role of hydrocarbons and nitrogen-containing species in the organic growth. 526
It is more or less admitted that Titan’s atmospheric aerosols may possess a polymeric 527
structure (at least for the aerosol nucleation step), but their structural basis remains unknown 528
because no instrument onboard Cassini provides a precise chemical information on the 529
32
aerosol composition. The only ACP instrument (Israel et al., 2005) of the Huygens probe 530
provides qualitative analysis of the global chemical content of the aerosols in Titan’s 531
atmosphere. It used pyrolysis to analyse the refractory nucleus of aerosols during its descent 532
through Titan’s atmosphere. The major pyrolysis products were HCN and NH3 showing that 533
Titan’s aerosols are in fact very nitrogen-rich, which excludes the commonly suggested 534
theory that these aerosols are derived from polycyclic aromatic hydrocarbons (PAHs). 535
PAMPRE tholins polymeric structure has been studied in Carrasco et al., 2009; Pernot et al., 536
2010, who confirm an important contribution of nitrogen in the aerosols composition as in 537
Titan’s aerosols; and its central role in the unsaturated component of tholins. 538
Nevertheless, a few studies on Titan’s atmosphere (Imanaka et Smith, 2007; Waite Jr. et al., 539
2007) and on pure methane and CH4-N2 plasmas (Deschenaux et al., 1999; Majumdar et al., 540
2005) emphasize a chemical growth carried by hydrocarbons (through ionic or neutral 541
processes) which could even been scavenged in the presence of nitrogen (Majumdar et al., 542
2005). 543
Those two hydrocarbon and nitrogenous pathways are not exclusive and possibly both 544
participate to the chemical blocks constitutive of Titan’s aerosols. Nevertheless, it would be 545
hazardous to reduce organic aerosol formation in the solar system to the only PAH theory just 546
because the nitrogen chemistry is far from being understood (see for example the recent paper 547
of Teanby et al., 2010 highlighting an unsuspected chemistry of nitriles). It is important to 548
remind that Quirico et al., 2008 found in an IR and Raman study of the PAMPRE tholins a 549
signature of nitrogen-rich aromatic rings: triazine (C3N3 aromatic ring), but none of benzene 550
aromatic ring. This was also confirmed by Gautier et al. (2011) where the analysis of the 551
PAMRE gas phase by GC-MS revealed that most abundant aromatics compounds are nitrogen 552
bearing ones, and that pure hydrocarbons aromatics are only minor compounds. Efforts on 553
33
this original N-PAH chemistry should be given in the future and could impact larger scales 554
than the specific Titan’s aerosols. 555
Our study confirms the importance of N-bearing compounds for the production of solid 556
particles simulating a Titan-like partially ionized N2-CH4 gas mixture: the PAMPRE reactive 557
plasma, ionized at the order of 10-2 ppm (Alcouffe et al., 2010), less than in Titan’s 558
ionosphere (0.1-1 ppm), is full of nitrogen-rich solid particles and exhibits a gas mixture 559
largely dominated by N-bearing gas products, including nitriles, ammonia and imines. 560
561
562
34
3.3. Positive ion species 563
Ammonia NH3 and methanimine CH2=NH are of great interest concerning prebiotic 564
chemistry, providing reactive nitrogen-containing species to Titan’s atmosphere. 565
In the ionosphere, the protonation, of methanimine followed by dissociative recombination of 566
the ion with electrons (reaction 11) may explain an additional production of NH2, and all the 567
more of ammonia in Titan ionosphere according to the following mechanism proposed by 568
Yelle et al. (2010). 569
N(2D)+CH4�CH2=NH+H (9) 570
CH2=NH+XH+�CH2=NH2
++X (10) 571
CH2=NH2++e-
�CH2+NH2 (11) 572
NH2+H2CN�NH3+HCN (12) 573
Methanimine is detected by mass analysis of the neutral species in our experiment for 574
[CH4]0=1% (see Section 3.2), but its signature at m/z=29 is hidden by a propane fragment for 575
the other methane conditions (see Figure 11). 576
In order to have a confirmation of this methanimine production at the larger concentration of 577
methane, we analyzed the positive ion study performed by Mutsukura, 2001 in a similar RF 578
plasma setup but working at lower pressure (0.13 mbar) than in our case. Methanimine 579
production can actually be inferred from the measurement of its protonated form CH2NH2+ 580
(unambiguous signature at m/z 30), as it has been done for Titan’s ionosphere with the ion 581
INMS Cassini spectra (Yelle et al., 2010). Mutsukura measured the ions characteristic of a 582
pure nitrogen, a pure methane and a 50% nitrogen-methane radio-frequency plasma. The mass 583
spectra are given in arbitrary units on Figure 12a. 584
585
586
35
10 15 20 25 30 35 40 45 50 55 600
102030405060708090
100
Inte
nsity
, a.u
m/z
CH5
+
10 15 20 25 30 35 40 45 50 55 600
200400600800
10001200140016001800
CH3CHNH
2
+
CH3CNH+
CH2NH
2
+
HCNH+
NH4
+
(b)
RF Plasma N2/CH
4 (50/50)
Plasma N2
Plasma CH4
Plasma N2/CH
4 (50/50)
INMS T19
Inte
nsity
, a.u
m/z
(a)
587
Figure 12: (a) Ion mass spectra measured in an RF plasma in pure nitrogen, pure methane and a 50-50 588
gas mixture and adapted from the study of Mutsukura, 2001. (b) Qualitative comparison of positive ion 589
mass spectra observed in a N2-CH4 RF plasma and in Titan’s ionosphere, in arbitrary units and 590
normalized at m/z 28. 591
592
36
Figure 12-b is a comparison of the ion mass spectra measured both in Titan’s ionosphere 593
(Waite Jr. et al., 2007) and in the RF-N2-CH4 plasma of Mutsukura, 2001. It shows a 594
qualitative agreement between the plasma setup and Titan’s ionosphere: the ions detected in 595
both ionized media are the same. However it also illustrates a quantitative difference: in the 596
Mutsukura’s RF plasma generates much more primary ions in comparison with heavier ions 597
than Titan’s ionosphere. Titan’s ionosphere on the contrary exhibits a larger proportion of 598
heavy ions. The difference can be explained by a more efficient production of primary ions in 599
the RF plasma, because the electron energy distribution function in the plasma has a longer 600
tail in the higher energies responsible for the primary ions production than the solar spectrum 601
(Szopa et al., 2006). In laboratory plasmas, positive ions and electrons are lost by 602
recombination and diffusion to the metallic walls. The primary ions are therefore less 603
available for ion-molecule reactions, processes responsible for the ion growth in Titan’s 604
ionosphere as shown in Carrasco et al., 2008. 605
As in the INMS ion mass spectra, an intense peak at m/z 30 is highlighted in the RF nitrogen-606
methane plasma which is absent in the pure and in the methane plasmas. Methanimine is 607
therefore a characteristic product of RF N2-CH4 plasmas. 608
609
3.3.1. Discussion on the methanimine production pathways 610
The simultaneous production of NH3 and methanimine is in agreement with the correlation 611
proposed by Yelle et al.(2010) for Titan’s ionosphere, but it is not clear whether ammonia is a 612
source for methanimine, or the opposite, as suggested by Yelle et al. Indeed different sources 613
can be found in the literature concerning the possible production pathways of these two 614
nitrogenous volatiles. Those are detailed below. 615
We identified two types of production pathways for methanimine compatible with Titan’s 616
ionosphere and with PAMPRE plasma chemistry: 617
• A radical pathway (Balucani et al., 2009) 618
N(2D)+CH4 � CH2=NH+H (13) 619
37
In this case, the positive correlation between methanimine production and methane 620
concentration is trivial. 621
• Ion chemistry pathways forming protonated methanimine, CH4N+, mainly involving 622
hydrocarbon ions with ammonia (Carrasco et al., 2008, based on the reference 623
database of Anicich, 2003) 624
o CH2+ + NH3 � CH4N
+ + H (14) 625
o CH3+ + NH3 � CH4N
+ + H2 (15) 626
o C3H6+ + NH3 � CH4N
+ + C2H5 (16) 627
o NH2+ + C2H4 � CH4N
+ + 1CH2 (17) 628
The rate constants of the above ion-molecule reactions are all in the same order of magnitude 629
of about 10-9 cm3.s-1 (Langevin rate constant for ion/molecule capture). However we can 630
suspect an important role of the reaction involving methyl ion CH3+ with ammonia, because 631
of the efficient production pathways of CH3+. This ion is indeed major in the nitrogen-632
methane plasma measured by Mutsukura, 2001 (Figure 12: a), and its production involves the 633
reactions of the two main nitrogen ions N2+ and N+ with methane. Moreover we can note that 634
CH3+ production is favoured by an increase of the methane concentration, which would 635
suggest an increase of the observed methanimine production with methane. The co-reactant 636
with CH3+ leading to methanimine is ammonia, which is also positively correlated with the 637
methane concentration through the N++CH4 reaction. 638
Methanimine ionic production pathways depend on the recombination of protonated 639
methanimine with electrons. There is however no data on the fragmentation pattern of this ion 640
by dissociative recombination with electrons (Plessis et al., 2010), to confirm that 641
methanimine could actually be the major product of such a recombination. It is precisely for 642
this reaction that Yelle et al., 2010 estimated two balanced pathways to explain ammonia 643
38
production in Titan’s ionosphere: a 0.5 H-loss branching ratio leading to methanimine 644
production, and a 0.5 dissociative branching ratio leading to NH2 production. 645
646
3.3.2. Impact of the detection of methanimine on Titan’s aerosol production 647
Methanimine is known as an easily polymerizable compound and is already suspected to take 648
part in Titan’s aerosol formation (Balucani et al., 2010). An oligomer of methanimine would 649
actually be in agreement with the C/N alternance detected in the paper of Carrasco et al., 2009 650
where a C2N3- common systematic feature was identified in the fragmentation pattern of all 651
the negative ions obtained after APPI ionization of tholins. It is also in agreement with the 652
result that all the double bonds in tholins are supported by C=N bonds, as shown in Pernot et 653
al., 2010; Somogyi et al., 2005. 654
Models describing the aerosols production and evolution in Titan’s atmosphere involve 655
polyynes, aromatics and nitrile pathways (Lavvas et al., 2008; Lebonnois et al., 2002). It 656
would be important for future work on Titan’s aerosols micro-physics to also implement this 657
newly discovered imine pathway. 658
659
39
4-Conclusion 660
The study of the stationary gas phase composition obtained with various initial methane 661
concentrations from 1 to 10%, confirms the enrichment of the gas phase - number of species 662
detected and global quantity - with the input of methane observed in Gautier et al., 2011. A 663
specific case is found at 1%, which almost only produces nitrogenous species and few 664
hydrocarbons. Atomic hydrogen is quantified and an anti-correlation is found between the 665
abundance of atomic hydrogen and the aerosol production efficiency. This result is in 666
agreement with the inhibiting heterogeneous effect suggested in Sciamma-O'Brien et al., 667
2010. A modelling of nitrogen-methane RF plasma, similar to the study of Pintassilgo et al., 668
1999 but which was dedicated to a N2-CH4 glow discharge, would be an essential 669
complementary tool enabling to interpret the correlations found here between volatiles and 670
aerosol production. 671
Moreover ammonia and methanimine are detected in the plasma in agreement with the 672
detection by INMS in Titan’s ionosphere (Yelle et al., 2010), such as ethanimine. The 673
production in the gas phase of such highly polymerizable species provides promising clues to 674
understand the production of tholins. 675
In the frame of Titan’s upper atmosphere, our study confirms two main antagonist effects on 676
aerosol production and growth. The first effect supports the organic growth process suggested 677
by Balucani et al. (2009, 2010) and involving a co-polymerization with imine species. The 678
second one is an inhibiting effect of atomic hydrogen on the aerosol production. This effect is 679
related to the observation of Sekine et al. (2008a, 2008b), showing an efficient fixation of 680
atomic hydrogen on aerosols. Here we show that the efficient hydrogen fixation is made at the 681
expense of the aerosol organic growth itself and has to be considered as a competition with 682
the aerosol organic growth process. 683
40
Acknowledgements 684
We thank L. Hogrel for her participation to the experimental work. We acknowledge the 685
CNRS (PID OPV, PNP), and the PRES UniverSud and the French National Research Agency 686
(ANR-09-JCJC-0038) for their financial support. 687
688
41
Figure 1: Blank mass spectrum of the mass spectrometer only. ................................................ 7 689
Figure 2: Comparison between a scan analog and a scan bargraph acquisition on the example 690
of the stationary state obtained after plasma discharge in a 1% methane initial gas mixture. ... 8 691
Figure 3: Typical example of emission spectrum obtained between 475 and 828 nm for 5% 692
CH4/N2 RF plasma with the presence of 5% Ar as an actinometer gas. The mean features 693
emissions are identified together their wavelengths. ............................................................... 11 694
Figure 4: Dependence of (a) Hβ (486.1 nm) and (b) Ar (811.5 nm) emission line intensities on 695
the percentage of CH4 added to N2 RF plasma. The percentage of Ar as an actinometer gas is 696
5% for all measurements. ......................................................................................................... 13 697
Figure 5: Evolution of hydrogen atom densities as a function of % CH4 obtained from Figure 698
4 in the same experimental conditions. .................................................................................... 14 699
Figure 6: Mass spectrum of a CH4-N2 10-90% gas mixture, (black) plasma OFF, (red) plasma 700
ON. ........................................................................................................................................... 16 701
Figure 7: Hydrogen production (difference between the stationary Iss(2) and the initial I0(2) 702
intensities of the mass peak at m/z=2) (above) according to the initial methane concentration 703
in the gas mixture: (below) according to the methane consumption (difference between the 704
stationary Iss(15) and the initial I0(15) intensities of the mass peak at m/z=15). ..................... 18 705
Figure 8: Intensity ratios I17/I18 as a function of the initial methane concentration (red) when 706
the plasma is ON, (black) when the plasma is OFF. The uncertainties calculated corresponds 707
to the 2σ statistical experimental scattering, calculated with 3-8 experiments when the plasma 708
is ON for each methane percentage, and with 30 spectra when the plasma is OFF (statistics on 709
the whole set of experiments, whatever the initial methane percentage). ................................ 21 710
Figure 9: Evolution of significant peaks detected, according to the initial methane amount. 711
Reported uncertainties are 2σ calculated on four repetitions of each spectrum and two 712
experiments per methane condition. ......................................................................................... 24 713
42
Figure 9: Differential mass spectra (Stationary spectrum – Initial spectrum) for initial gas 714
mixtures of (black) 1%, (blue) 5% and (red) 10% of methane in the initial gas mixture. ....... 26 715
Figure 11: Evolution of the ratio between the increases of m/z 44 and m/z 29 when the plasma 716
is ON. The stable value of about 2 for [CH4]0 concentration larger than 2% is in agreement 717
with a co-evolution of fragments intensities for propane. However, the larger value of this 718
ratio for [CH4]0=1% indicates an additional contribution of methanimine at mass 29 u. ........ 29 719
Figure 12: (a) Ion mass spectra measured in an RF plasma in pure nitrogen, pure methane and 720
a 50-50 gas mixture and adapted from the study of Mutsukura, 2001. (b) Qualitative 721
comparison of positive ion mass spectra observed in a N2-CH4 RF plasma and in Titan’s 722
ionosphere, in arbitrary units and normalized at m/z 28. ......................................................... 35 723
724
725
726
727
728
729
730
43
References 731
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