194 research outputs found
Characterization of aromaticity in analogues of titan's atmospheric aerosols with two-step laser desorption ionization mass spectrometry
The role of polycyclic aromatic hydrocarbons (PAH) and Nitrogen containing
PAH (PANH) as intermediates of aerosol production in the atmosphere of Titan
has been a subject of controversy for a long time. An analysis of the
atmospheric emission band observed by the Visible and Infrared Mapping
Spectrometer (VIMS) at 3.28 micrometer suggests the presence of neutral
polycyclic aromatic species in the upper atmosphere of Titan. These molecules
are seen as the counter part of negative and positive aromatics ions suspected
by the Plasma Spectrometer onboard the Cassini spacecraft, but the low
resolution of the instrument hinders any molecular speciation.
In this work we investigate the specific aromatic content of Titan's
atmospheric aerosols through laboratory simulations. We report here the
selective detection of aromatic compounds in tholins, Titan's aerosol
analogues, produced with a capacitively coupled plasma in a N2:CH4 95:5 gas
mixture. For this purpose, Two-Step Laser Desorption Ionization Time-of-Flight
Mass Spectrometry (L2DI-TOF-MS) technique is used to analyze the so produced
analogues. This analytical technique is based on the ionization of molecules by
Resonance Enhanced Multi-Photon Ionization (REMPI) using a {\lambda}=248 nm
wavelength laser which is selective for aromatic species. This allows for the
selective identification of compounds having at least one aromatic ring. Our
experiments show that tholins contain a trace amount of small PAHs with one to
three aromatic rings. Nitrogen containing PAHs (PANHs) are also detected as
constituents of tholins. Molecules relevant to astrobiology are detected as is
the case of the substituted DNA base adenine
Characterization of a DC glow discharge in N2-H2 with electrical measurements and neutral and ion mass spectrometry
The addition of small amounts of H2 were investigated in a DC glow discharge
in N2, at low pressure (~1 mbar) and low power (0.05 to 0.2 W.cm-3). We
quantified the electric field, the electron density, the ammonia production and
the formation of positive ions for amounts of H2 varying between 0 and 5%,
pressure values between 0.5 and 4 mbar, and currents between 10 and 40 mA. The
addition of less than 1% H2 has a strong effect on the N2 plasma discharges.
Hydrogen quenches the (higher) vibrational levels of N2 and some of its highly
energetic metastable states. This leads to the increase of the discharge
electric field and consequently of the average electron energy. As a result,
higher quantities of radical and excited species are suspected to be produced.
The addition of hydrogen also leads to the formation of new species. In
particular, ammonia and hydrogen-bearing ions have been observed: N2H+ and NH4+
being the major ones, and also H3+, NH+, NH2+, NH3+, N3H+ and N3H3+. The
comparison to a radiofrequency capacitively coupled plasma (RF CCP) discharge
in similar experimental conditions shows that both discharges led to similar
observations. The study of N2-H2 discharges in the laboratory in the adequate
ionization conditions then gives some insights on which plasma species made of
nitrogen and hydrogen could be present in the ionosphere of Titan. Here, we
identified some protonated ions, which are reactive species that could
participate to the erosion of organic aerosols on Titan.Comment: Paper accepted in Plasma Sources Science and Technology in March
2023. The current version on arXiv is the submitted versio
Experiments and modelling in N2-H2 capacitively coupled radio-frequency discharges at low pressure
This work uses experiments and simulations to analyze the modifications induced in pure N2 capacitively coupled radio-frequency discharges, running at low pressure (0.6–1.2 mbar) and low power (5–20 W), by the addition of small amounts of H2 (up to 5%). Simulations use a hybrid code coupling a two-dimensional time-dependent fluid module, describing the dynamics of the charged particles, to a zero-dimensional kinetic module, describing the production and destruction of nitrogen and hydrogen neutral species. The discussion is particularly focused on the results obtained for the electron density and the radiative transition intensities with nitrogen species. Model predictions are in qualitative agreement with measurements, for the evolution of these quantities with changes in both the gas pressure and the hydrogen percentage in the gas mixture.Fundação para a Ciência e a Tecnologia (FCT
Gaseous chemistry for a Titan's atmospheric plasma experimental simulation
We present the first study of gaseous composition monitoring for the PAMPRE experiment, which simulates Titan's atmospheric chemistry by radio-frequency N 2-CH 4 plasma. Methane consumption is quantified for various N 2-CH 4 gas mixtures. Moreover in situ mass spectrometry (MS) and ex-situ gas chromatography coupled with mass spectrometry (GC-MS) analyses reveal a large dominance of nitrile species in the gas phase chemistry
N2-H2 capacitively coupled radio-frequency discharges at low pressure: II. Modeling results: The relevance of plasma-surface interaction
In this work, we present the results of simulations carried out for N2-H2 capacitively coupled radio-frequency discharges, running at low pressure (0.3-0.9 mbar), low power (5-20 W), and for amounts of H2 up to 5%. Simulations are performed using a hybrid code that couples a two-dimensional time-dependent fluid module, describing the dynamics of the charged particles in the discharge, to a zero-dimensional kinetic module, that solves the Boltzmann equation and describes the production and destruction of neutral species. The model accounts for the production of several vibrationally and electronic excited states, and contains a detailed surface chemistry that includes recombination processes and the production of NH x molecules. The results obtained highlight the relevance of the interactions between plasma and surface, given the role of the secondary electron emission in the electrical parameters of the discharge and the critical importance of the surface production of ammonia to the neutral and ionic chemistry of the discharge.The Portuguese Foundation sponsored this research for Science and Technology (FCT) in the framework of the
Strategic Funding UID/FIS/04650/2019
Modelling of a CCP-RF discharge used for the simulation of Titan’s chemistry
This paper reports the modelling of CCP-RF discharges (13.56 MHz) in pure nitrogen, produced within a cylindrical parallel-plate reactor, similar to a GEC reference cell surrounded by a lateral grounded grid, at 0.1-2 mbar pressures and 10-50 W coupled powers. This study is a first step in simulating Titan’s chemistry at laboratory scale, using the PAMPRE experiment. Modelling results are compared with experimental measurements of the average electrondensity, and the self-bias potential at the polarized electrode
Capacitively coupled radio-frequency N2 discharges at low pressures
Capacitively coupled radio-frequency discharges (ccrf) in nitrogen mixtures are frequently used for
the processing, modification and functionalization of different kinds of materials. Although nitrogen
plasmas have been studied for many years, and despite their growing interest in applications, there is
only partial knowledge about ccrf nitrogen plasmas.
This paper uses experiments and modelling to study ccrf discharges in pure nitrogen, at 13.56 MHz
frequency, 0.1–1 mbar pressures and 2–30 W coupled powers [1]. Experiments performed on two
similar (not twin) setups, existing in the LATMOS and the GREMI laboratories, include electrical and
optical emission spectroscopy (OES) measurements. Electrical measurements give the rf-applied and
the direct-current-self-bias voltages, the effective power coupled to the plasma and the average
electron density. OES diagnostics measure the intensities of radiative transitions with the nitrogen
second-positive and first-negative systems, and with the 811.5 nm atomic line of argon (present as an
actinometer).
In the particular case of non-equilibrium ccrf discharges in nitrogen, a self-consistent modeling
strategy must account for the interplay between the transport of particles, in the presence of density
gradients and the rf field, and their production/destruction due to kinetic mechanisms involving both
electrons and heavy species. Simulations use a hybrid code that couples a two-dimensional timedependent
fluid module [2], describing the dynamics of the charged particles (electrons and positive
ions N2
+ and N4
+), and a zero-dimensional kinetic module, describing the production and destruction
of nitrogen (atomic and molecular) neutral species [3]. The coupling between these modules adopts
the local mean energy approximation to define space–time-dependent electron parameters for the fluid
module and to work out space–time-averaged rates for the kinetic module. The model gives general
good predictions for the self-bias voltage and for the intensities of radiative transitions (both average
and spatially resolved), underestimating the electron density by a factor of 3–4.Fundação para a Ciência e a Tecnologia (FCT
Use of the linear absorption coefficient for absolute comparison of plasma films in the mid-IR range
In this work, we present the mid-infrared analysis of analogues of Titan's aerosols produced in a radio frequency capacitively coupled plasma (RF-CCP). The influence of the gas mixture on aerosols spectra is also studied through the analysis of the carbonaceous bands of the spectra, and its Gaussian deconvolution.Представлен анализ в среднем инфракрасном диапазоне аналогов титановых аэрозолей, которые производятся в радиочастотных источниках плазмы с емкостной связью (РЧ-ЕСС). Также изучено влияние газовой смеси на спектры аэрозолей с помощью анализа углеродсодержащих полос спектра и ее гауссовой деконволюции (обратная свертка).Представлено аналіз у середньому інфрачервоному діапазоні аналогів титанових аерозолів, які проводяться в радіочастотних джерелах плазми з ємнісним зв'язком (РЧ-ЕСС). Також вивчено вплив газової суміші на спектри аерозолів за допомогою аналізу вуглецевмісних смуг спектра і її гауссової деконволюції (зворотня згортка)
Use of the linear absorption coefficient for absolute comparison of plasma films in the mid-IR range
In this work, we present the mid-infrared analysis of analogues of Titan's aerosols produced in a radio frequency capacitively coupled plasma (RF-CCP). The influence of the gas mixture on aerosols spectra is also studied through the analysis of the carbonaceous bands of the spectra, and its Gaussian deconvolution.Представлен анализ в среднем инфракрасном диапазоне аналогов титановых аэрозолей, которые производятся в радиочастотных источниках плазмы с емкостной связью (РЧ-ЕСС). Также изучено влияние газовой смеси на спектры аэрозолей с помощью анализа углеродсодержащих полос спектра и ее гауссовой деконволюции (обратная свертка).Представлено аналіз у середньому інфрачервоному діапазоні аналогів титанових аерозолів, які проводяться в радіочастотних джерелах плазми з ємнісним зв'язком (РЧ-ЕСС). Також вивчено вплив газової суміші на спектри аерозолів за допомогою аналізу вуглецевмісних смуг спектра і її гауссової деконволюції (зворотня згортка)
N2-H2 capacitively coupled radio-frequency discharges at low pressure. Part I. Experimental results: Effect of the H2 amount on electrons, positive ions and ammonia formation
The mixing of N2 with H2 leads to very different plasmas from pure N2 and H2 plasma discharges. Numerous issues are therefore raised involving the processes leading to ammonia (NH3) formation. The aim of this work is to better characterize capacitively-coupled radiofrequency plasma discharges in N2 with few percents of H2 (up to 5%), at low pressure (0.3-1 mbar) and low coupled power (3-13 W). Both experimental measurements and numerical simulations are performed. For clarity, we separated the results in two complementary parts. The actual one (first part), presents the details on the experimental measurements, while the second focuses on the simulation, a hybrid model combining a 2D fluid module and a 0D kinetic module. Electron density is measured by a resonant cavity method. It varies from 0.4 to 5 109 cm-3, corresponding to ionization degrees from 2 10-8 to 4 10-7. Ammonia density is quantified by combining IR absorption and mass spectrometry. It increases linearly with the amount of H2 (up to 3 1013 cm-3 at 5% H2). On the contrary, it is constant with pressure, which suggests the dominance of surface processes on the formation of ammonia. Positive ions are measured by mass spectrometry. Nitrogen-bearing ions are hydrogenated by the injection of H2, N2H+ being the major ion as soon as the amount of H2 is >1%. The increase of pressure leads to an increase of secondary ions formed by ion/radical-neutral collisions (ex: N2H+, NH4 +, H3 +), while an increase of the coupled power favours ions formed by direct ionization (ex: N2 +, NH3 +, H2 +).N. Carrasco acknowledges the financial support of the European Research Council (ERC Starting Grant
PRIMCHEM, Grant agreement no. 636829).
A. Chatain acknowledges ENS Paris-Saclay Doctoral Program. A. Chatain is grateful to Gilles Cartry and
Thomas Gautier for fruitful discussions on the MS calibration.
L.L. Alves acknowledges the financial support of the Portuguese Foundation for Science and Technology (FCT) through the project UID/FIS/50010/2019.
L. Marques and M. J. Redondo acknowledge the financial support of the Portuguese Foundation for Science
and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2019
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