Diagnóstico y modelado de plasmas fríos con alto contenido en hidrógeno

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física Atómica, Molecular y Nuclear, leída el 15/11/2016The study of cold plasmas with high hydrogen content is relevant in a wide variety of fields. The characterization of the different species in these plasmas and their interaction with surfaces has technological applications, such as thin film growth and materials processing. The ions present in this type of plasmas play an important role in molecule formation in the interstellar medium. In this work, a study of the neutral and ionic species present in plasmas of H2 + Ar,H2 + N2, and H2 + O2 generated in hollow cathode discharges is presented. Experimentaldiagnostics in combination with kinetic modeling have been employed in order to determine the main processes behind the physics and chemistry of each of the different mixtures. The hollow cathode reactor used in the experiments consists of a grounded stainless steel cylinder with a central anode. A voltage of hundreds of volts is applied to the anode, generating a current of tenths of an ampere. A steady flow of mixtures of H2 + Ar, H2 + N2,and H2 + O2 with different mixture ratios at low pressures (~ Pa) is employed as precursor. The neutral species in the plasma are analyzed with a quadrupole mass spectrometer, while positive ions are sampled using a plasma monitor that allows themeasurement of their energy distributions. The electron temperature and density of theplasma are determined using a double Langmuir probe. Vibrational and rotational gas temperatures are obtained by optical emission spectroscopy. A kinetic model accounting for the main physico-chemical processes occurring in the discharge is employed in order to simulate the abundances of the different species. Thecomparison with the experimental results allows the determination of the fundamental mechanisms behind the chemistry of the different mixtures. The main processes considered are electron impact dissociation and ionization, ion-molecule reactions, and heterogeneous reactions occurring at the reactor walls...El estudio de los plasmas fríos con alto contenido en hidrógeno tiene interés en una gran variedad de campos. La caracterización de las especies en estos plasmas y su interacción con las superficies tiene aplicaciones tecnológicas, como el crecimiento de láminas delgadas y el procesado de materiales. Los iones que se pueden observar en este tipo de plasmas tienen un papel importante en la formación de moléculas en el medio interestelar. En este trabajo se presenta un estudio de las especies neutras e iónicas presentes en los plasmas de H2 + Ar, H2 + N2, y Hi + 02 generados en descargas de cátodo hueco, basado en el diagnóstico experimental y el modelado cinético, con el objetivo de determinar los principales procesos físico-químicos de cada una de las mezclas. El reactor de cátodo hueco empleado en los experimentos consiste en un cilindro de acero inoxidable conectado a tierra, con un ánodo central sometido a una tensión de cientos de voltios, donde se genera una corriente de décimas de amperio. Para generar el plasma se emplean mezclas de H2 + Ar, Hz+ N2, y Hz+ 02 en diferentes proporciones, a baja presión ( ~ Pa), en flujo continuo de gas. Las especies neutras del plasma se analizan mediante un espectrómetro de masas cuadrupolar, mientras que los iones positivos se muestrean mediante un monitor de plasmas que permite analizar sus distribuciones de energía. La temperatura y densidad electrónica del plasma se determinan usando una sonda de Langmuir doble. Las temperaturas vibracional y rotacional se obtienen mediante espectroscopía visible de emisión. Con el objetivo de reproducir las abundancias de las diferentes especies determinadas experimentalmente en cada tipo de mezcla y determinar los mecanismos que gobiernan la cinética del plasma, se emplea un modelo cinético que tiene en cuenta los principales procesos físicoquímicos que tienen lugar en la descarga. Entre ellos, los más relevantes son las reacciones de ionización y disociación por impacto electrónico y las reacciones ion-molécula, que tienen lugar en fase gas, y los procesos heterogéneos que ocurren en las paredes de la cámara...Depto. de Estructura de la Materia, Física Térmica y ElectrónicaFac. de Ciencias FísicasTRUEunpu

Energy distributions of neutrals and ions in H2 low temperature plasmas: a study of fast H atoms

In this work we study by visible emission spectroscopy, energy resolved ion mass spectrometry and electric probes, H2 plasmas generated in low pressure hollow cathode glow discharges. The study allows the determination of the energy distributions of the different plasma constituents (ions, electrons and neutrals), which span five orders of magnitude. The rotational H2 temperature, assumed to be close to the translational one, scarcely exceeds the room temperature (0.03 eV); free electrons, responsible of primary ionizations and dissociations, display mean kinetic energies of 3-6 eV; whereas ions (H+, H2+, H3+) and a part of H atoms reach the highest energies (300 eV). Our present study focuses on the line-shape analysis of the H Balmer series emitted by the plasma, whose spectral profiles evince its remarkable deviation from thermal equilibrium, and on the dependence of these profiles with H2 pressure. The aim is to get a deeper understanding of the processes responsible of this behaviour. Three different Doppler broadenings are found in the atomic lines: the narrow peak, the plateau, and the far wings, with FWHM of ~ 0.3, 6 and up to 80 eV, respectively [1]. Besides, a directional and asymmetric behaviour in the line shifts up to some 300 eV is observed when ions are directed preferentially towards the observation window through a grounded grid. The narrow line peak can be explained by direct electron impact excitation of the free H atoms and by Frank-Condon transitions to Rydberg levels. The plateau is also explained by electronic excitations, in this case to pre-dissociative levels and levels giving rise to dissociative ionization. Their effectiveness depends primarily on electron temperature. In contrast, the far wings are assumed to be mainly due to charge transfer processes and dissociative reactions of H2 with the H+, H2+ and H3+ ions accelerated in the plasma sheath. The latter processes depend strongly on collision frequency and increase appreciably with pressure. The ion energy distributions obtained by mass spectrometry [2, 3] and the electron temperatures measured with the electric probe support the validity of the proposed mechanisms.Peer Reviewe

Chemistry in glow discharges of H2/O2 mixtures. Diagnostics and modelling

Peer Reviewe

Neutral and ion kinetics in glow discharges of H2/O2 mixtures. Diagnostics and modelling

A combined diagnostics and modelling of low pressure H2/O2 plasmas at different pressures and mixture ratios, generated in a low pressure hollow cathode DC reactor, is presented. Neutral and ion distributions are measured by mass spectrometry. Langmuir probes provide charge densities and electron temperatures. As expected, apart from the precursors, H2O is detected in considerable amounts and H3O+ is a major ion for H-rich mixtures. A zero order kinetic model is used to explain the experimental results. H2O is produced via plasma-surface interactions in a multistep process. The ion distributions are determined in each case by a balance between the relative weights of electron impact processes and proton transfer chemistry.N

Neutral and ion kinetics in glow discharges of H2 / O2 mixtures. Diagnostics and modelling

London, United Kingdom, 14–17 April 2014; http://plasma14.iopconfs.org/homeLow pressure plasmas in electrical discharges with H2 and O2 are of interest in a variety of fields. In astrochemistry, the formation of H2O and H3O+ is of great relevance as they can be detected in interstellar environments. In fusion research, discharge cleaning is used to eliminate the residual molecules in a vacuum vessel, of which oxygen and water are major components. In this work, we present a study of the chemistry of neutral and ionic species in H2/O2 plasmas based on the experimental diagnostics and kinetic modelling of hollow cathode discharges at different pressures and mixture proportions. Neutral and ion distributions are measured by mass spectrometry, and Langmuir probes provide charge densities and electron temperatures. As expected, apart from the precursors, H2O is detected in considerable amounts. Concerning the charged species, pure hydrogen and oxygen ions are detected together with mixed ones. With increasing pressure, the ion distributions are dominated by H3O+ for mixtures with H2 concentrations higher than ~ 30%, in contrast, the protonated species O2H+ is hardly formed. A zero order kinetic model is used to explain the experimental results. H2O is produced via plasma-surface interactions in a multistep process. The ion distributions are determined in each case by a balance between the relative weights of electron impact processes and proton transfer chemistry. The relevance of negative ions in the plasma chemistry is also studied.Peer Reviewe

Chemistry in glow discharges of H2/O2 mixtures. Diagnostics and modelling

Peer Reviewe

High energy electron irradiation of interstellar carbonaceous dust analogs: Cosmic ray effects on the carriers of the 3.4 μm absorption band

9 págs; 5 figs.; 6 tabs.The effects of cosmic rays on the carriers of the interstellar 3.4 μm absorption band have been investigated in the laboratory. This band is attributed to stretching vibrations of CH3 and CH2 in carbonaceous dust. It is widely observed in the diffuse interstellar medium, but disappears in dense clouds. Destruction of CH3 and CH2 by cosmic rays could become relevant in dense clouds, shielded from the external ultraviolet field. For the simulations, cules, formed by the recombination of H atoms liberated through CH bond breaking, diffuse out of the sample. The CH bond destruction rates derived from the present experiments are in good accordance with those from previous ion irradiation experiments of HAC. The experimental simplicity of electron the 3.4 μm band carriers lie in the 108 yr range and cannot account for the disappearance of this band in dense clouds, which have characteristic lifetimes of 3×107 yr. The results invite a more detailed investigation of the mechanisms of CH bond formation and breaking in the intermediate region between diffuse and dense clouds.This work has been funded by the MINECO of Spain under grant FIS2013-48087-C2-1P, by the MICINN of Spain under grant CDS2009-00038, and by the European project ERC-2013-Syg 610256. G.M. acknowledges MINECO PhD grant BES-2014-069355.Peer reviewe

Diagnostics and modeling of cold laboratory plasmas with high hydrogen content; applications to molecular astrophysics.

First General Meeting in Prague, May 25 - 29, 2015; http://prague2015astrohistory.vscht.cz/Cold plasmas of molecular precursors produced in low pressure glow discharges involve intricate mechanisms of great interest in many scientific and technological fields, such as thin film growth, surface conditioning and so on. In particular, hydrogen containing cold plasmas are currently used in microelectronic devices production, can simulate the border conditions of plasmas generated in fusion reactors, and are efficient sources of very reactive radicals and ions, whose study contributes to clarify the appearance of complex compounds in different regions of interstellar molecular clouds or in Jovian planet ionospheres. In this work, low pressure glow discharges of mixtures of hydrogen with simple molecules and atoms and different isotopic compositions are experimentally diagnosed, and the main mechanisms controlling their behavior are elucidated by kinetic modeling. According to theoretical predictions supported by experimental data, the formation of new molecular species takes place mainly at the reactor surfaces and competes with the fast wall recycling of the precursors, which are previously dissociated by electron impact. On the other hand, the ion distributions result mainly from the balance between electron impact ionizations, which depend markedly on electron energies, and ion-molecule reactions in gas phase, for which the proton affinity of the different neutral species plays a key role. The first detection of the deuterated ammonium ion (NH3D+) in the interstellar medium [1,2], and the refined infrared spectroscopic characterization of the isotopomers 36ArH+ and 36ArH+ [3], the first noble gas molecules found in space [4,5], exemplify the successful interaction between plasma spectroscopy and astronomical observations.The authors acknowledge the financial support from the Spanish MINECO through grants SD2009-00038, FIS2012- 38175 and FIS2013-48087-C2-1-P. Additional funding from ERC-2013-Syg 610256-NANOCOSMOS is also aknowledged.Peer Reviewe

Structure and infrared spectra of hydrocarbon interstellar dust analogs

9 págs.; 6 figs.; 5 tabs.A theoretical study of the structure and mid infrared (IR) spectra of interstellar hydrocarbon dust analogs is presented, based on DFT calculations of amorphous solids. The basic molecular structures for these solids are taken from two competing literature models. The first model considers small aromatic units linked by aliphatic chains. The second one assumes a polyaromatic core with hydrogen and methyl substituents at the edges. The calculated spectra are in reasonably good agreement with those of aliphatic-rich and graphitic-rich samples of hydrogenated amorphous carbon (HAC) generated in our laboratory. The theoretical analysis allows the assignment of the main vibrations in the HAC spectra and shows that there is a large degree of mode mixing. The calculated spectra show a marked dependence on the density of the model solids, which evinces the strong influence of the environment on the strengths of the vibrational modes. The present results indicate that the current procedure of estimating the hydrogen and graphitic content of HAC samples through the decomposition of IR features into vibrational modes of individual functional groups is problematic owing to the mentioned mode mixing and to the difficulty of assigning reliable and unique band strengths to the various molecular vibrations. Current band strengths from the literature might overestimate polyaromatic structures. Comparison with astronomical observations suggests that the average structure of carbonaceous dust in the diffuse interstellar medium lies probably in between those of the two models considered, though closer to the more aliphatic structure. ©the Owner Societies 2017This work has been funded by the MINECO of Spain under grant FIS2013-48087-C2-1P, and AYA2015-71975-REDT, by the MICINN of Spain under grant CDS2009-00038, and by the European project ERC-2013-SyG, Grant Agreement 610256 ‘‘NANOCOSMOS’’. G. M acknowledges MINECO PhD grant BES-2014-069355.Peer reviewe