Large effects of small pressure changes in the kinetics of low pressure glow discharges

Tanarro, Isabel; Herrero, Vı́ctor J.

doi:10.1088/0963-0252/20/2/024006

Cited by 13 publications

(15 citation statements)

References 38 publications

(70 reference statements)

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“…The reproducibility of the experiments suggests, however, that the surface is not appreciably modified by the studied discharges. As found previously in plasmas of other gas mixtures 75 , the overall ion results can be explained mainly by the decrease of the electron temperature T e at higher pressures. NH 4 + together with H 3 + and N 2 H + are formed exclusively by ion-neutral gas phase reactions (see Table 2).…”

Section: Resultssupporting

confidence: 79%

Neutral and ion chemistry in low pressure dc plasmas of H2/N2 mixtures: routes for the efficient production of NH3 and NH4+

Carrasco

Jiménez-Redondo

Tanarro

et al. 2011

Phys. Chem. Chem. Phys.

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125

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The chemistry in low pressure (0.8-8 Pa) plasmas of H(2) + 10% N(2) mixtures has been experimentally investigated in a hollow cathode dc reactor using electrical probes for the estimation of electron temperatures and densities, and mass spectrometry to determine the concentration of ions and stable neutral species. The analysis of the measurements by means of a kinetic model has allowed the identification of the main physicochemical mechanisms responsible for the observed distributions of neutrals and ions and for their evolution with discharge pressure. The chemistry of neutral species is dominated by the formation of appreciable amounts of NH(3) at the metallic walls of the reactor through the successive hydrogenation of atomic nitrogen and nitrogen containing radicals. Both Eley-Rideal and Langmuir-Hinshelwood mechanisms are needed in the chain of hydrogenation steps in order to account satisfactorily for the observed ammonia concentrations, which, in the steady state, are found to reach values ~30-70% of those of N(2). The ionic composition of the plasma, which is entirely due to gas-phase processes, is the result of a competition between direct electron impact dissociation, more relevant for high electron temperatures (lower pressures), and ion-molecule chemistry that prevails for the lower electron temperatures (higher pressures). At the lowest pressure, products from the protonation of the precursor molecules (H(3)(+), N(2)H(+) and NH(4)(+)) and others from direct ionization (H(2)(+) and NH(3)(+)) are found in comparable amounts. At the higher pressures, the ionic distribution is largely dominated by ammonium. It is found that collisions of H(3)(+), NH(3)(+) and N(2)H(+) with the minor neutral component NH(3) are to a great extent responsible for the final prevalence of NH(4)(+).

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Section: Resultssupporting

confidence: 79%

Neutral and ion chemistry in low pressure dc plasmas of H2/N2 mixtures: routes for the efficient production of NH3 and NH4+

Carrasco

Jiménez-Redondo

Tanarro

et al. 2011

Phys. Chem. Chem. Phys.

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125

240

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“…With the aim of investigating isotope exchange processes in room temperature hydrogen plasmas, we have carried out a detailed diagnostics and modelling of hollow cathode (HC) discharges of H 2 , D 2 , and H 2 /D 2 mixtures. The present study extends significantly our previous works on glow discharges of pure H 2 [44,45]. Over the pressure range studied (1-8 Pa) large changes in the composition of the plasma are observed and the key physicochemical processes responsible for these composition changes are identified with the help of a simple model that provides a good overall picture of the discharge kinetics.…”

Section: Introductionsupporting

confidence: 78%

Isotopic exchange processes in cold plasmas of H2/D2 mixtures

Jiménez-Redondo

Carrasco

Herrero

et al. 2011

Phys. Chem. Chem. Phys.

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Isotope exchange in low pressure cold plasmas of H(2)/D(2) mixtures has been investigated by means of mass spectrometric measurements of neutrals and ions, and kinetic model calculations. The measurements, which include also electron temperatures and densities, were performed in a stainless steel hollow cathode reactor for three discharge pressures: 1, 2 and 8 Pa, and for mixture compositions ranging from 100% H(2) to 100% D(2). The data are analyzed in the light of the model calculations, which are in good global agreement with the experiments. Isotope selective effects are found both in the surface recombination and in the gas-phase ionic chemistry. The dissociation of the fuel gas molecules is followed by wall recycling, which regenerates H(2) and D(2) and produces HD. Atomic recombination at the wall is found to proceed through an Eley-Rideal mechanism, with a preference for reaction of the adsorbed atoms with gas phase D atoms. The best fit probabilities for Eley-Rideal abstraction with H and D are: γ(ER H) = 1.5 × 10(-3), γ(ER D) = 2.0 × 10(-3). Concerning ions, at 1 Pa the diatomic species H(2)(+), D(2)(+) and HD(+), formed directly by electron impact, prevail in the distributions, and at 8 Pa, the triatomic ions H(3)(+), H(2)D(+), HD(2)(+) and D(3)(+), produced primarily in reactions of diatomic ions with molecules, dominate the plasma composition. In this higher pressure regime, the formation of the mixed ions H(2)D(+) and HD(2)(+) is favoured in comparison with that of H(3)(+) and D(3)(+), as expected on statistical grounds. The model results predict a very small preference, undetectable within the precision of the measurements, for the generation of triatomic ions with a higher degree of deuteration, which is probably a residual influence at room temperature of the marked zero point energy effects (ZPE), relevant for deuterium fractionation in interstellar space. In contrast, ZPE effects are found to be decisive for the observed distribution of monoatomic ions H(+) and D(+), even at room temperature. The final H(+)/D(+) ratio is determined to a great extent by proton (and deuteron) exchange, which favours the enhancement of H(+) and the concomitant decrease of D(+).

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“…Ion densities were calculated but only compared to uncalibrated signal intensities of the ion species from Gudmundsson 19,20 . Comparable quantitative comparisons between experimental and modeling results were previously published for hollow cathode dc discharges in H 2 -Ar mixtures by Méndez and coworkers 21,22 . Their glow discharge is operated at a high voltage of several hundred volts and the sheath is relatively thick.…”

Section: Introductionsupporting

confidence: 74%

Ion chemistry in H2-Ar low temperature plasmas

Sode

Schwarz-Selinger

Jacob

2013

Journal of Applied Physics

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A rate equation model is devised to study the ion composition of inductively coupled H 2 -Ar plasmas with different H 2 -Ar mixing ratios. The model is applied to calculate the ion densities n i , the wall loss probability of atomic hydrogen β H , and the electron temperature T e . The calculated n i 's of Ar + , H + , H + 2 , H + 3 and ArH + are compared with experimental results. Calculations were made for a total gas pressure of 1.0 Pa. The production and loss channels of all ions are presented and discussed in detail. With the production and loss rates the density dependence of each ion on the plasma parameters are explained. It is shown that the primary ions H + 2 and Ar + which are produced by ionization of the background gas by electron collisions are effectively converted into H + 3 and ArH + . The high density of ArH + and Ar + is attributed to the low loss to the walls compared to hydrogen ions. It is shown that the H + /H + 2 density ratio is strongly correlated to the H/H 2 density ratio. The dissociation degree is around 1.7 %. From matching the calculated to the measured atomic hydrogen density n H the wall loss probability of atomic hydrogen on stainless steel β H was determined to be β H = 0.24. The model results were compared with recently published experimental results. The calculated and experimentally obtained data are in fair agreement.

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Large effects of small pressure changes in the kinetics of low pressure glow discharges

Cited by 13 publications

References 38 publications

Neutral and ion chemistry in low pressure dc plasmas of H2/N2 mixtures: routes for the efficient production of NH3 and NH4+

Neutral and ion chemistry in low pressure dc plasmas of H2/N2 mixtures: routes for the efficient production of NH3 and NH4+

Isotopic exchange processes in cold plasmas of H2/D2 mixtures

Ion chemistry in H2-Ar low temperature plasmas

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