Flowing Afterglow Plasma Research Articles

Communication: Transfer ionization in a thermal reaction of a cation and anion: Ar+ with Br− and I−

We present experimental evidence that reactions of argon cations Ar(+) with the halogen anions Br(-) and I(-) do not occur exclusively by mutual neutralization, but also produce the cations Br(+) or I(+) ions by transfer ionization (TI). The experiments were carried out in flowing-afterglow plasmas at gas temperatures between and 300 and 500 K, and employed a variant of the Variable Electron and Neutral Density Attachment Mass Spectrometry method. The measured TI rate coefficients are 1.9 ± 0.6 × 10(-9) cm(3) s(-1) and 1.1 ± (0.3)(0.8) × 10(-9) cm(3) s(-1) for the Br(-) and I(-) reactions, respectively. We find that the TI rate coefficients decline with temperature as T(-0.5) to T(-1). No indication of TI was found in the reaction with Cl(-), where it is endoergic.

Plasma polymerized ferrocene films

This communication describes the formation of high index of refraction polymer thin films using a novel plasma polymerization deposition process. A flowing afterglow plasma reactor was modified to enable sublimation of solid samples into the gas phase for subsequent plasma polymerization. Thin films of plasma polymerized ferrocene were deposited on substrates and subsequently characterized. The refractive index as a function of processing conditions was obtained. Relatively high values of n (∼1.73 at 589nm) were obtained. The chemical nature of the polymer thin films was characterized using FTIR and XPS spectroscopy. This work demonstrates that plasma polymerization is an enabling technology for the fabrication of photonic thin films that utilize solid state precursors.

Yield of excited CO molecules from dissociative recombination of HCO+ and HOC+ ions with electrons

The authors have investigated CO band emissions arising from the dissociative recombination of HCO(+) and HOC(+) ions with thermal electrons in a flowing afterglow plasma. The quantitative analysis of the band intensities showed that HCO(+) recombination forms the long-lived CO(a (3)Pi) state with a yield of 0.23+/-0.12, while HOC(+) recombination favors formation of CO(a' (3)Sigma(+)) and CO(d (3)Delta) with a combined yield of greater than 0.4. The observed vibrational distribution for the CO(a) state reproduces theoretical predictions quite well. The vibrational distributions for CO(a') and CO(d) are, in part, inverted, presumably as a consequence of a change in CO equilibrium bond length during recombination. The observations are compatible with current knowledge of the potential surfaces of states of HCO and HCO(+).

Recombination of polycyclic aromatic hydrocarbon photoions with electrons in a flowing afterglow plasma

A new technique, flowing afterglow with photoions (FIAPI), has been developed to measure the rate coefficient for the recombination of complex ions, and, in particular, polycyclic aromatic hydrocarbon (PAH) cations with electrons. The method is based on the flowing afterglow Langmuir probe - mass spectrometer apparatus at the University of Rennes I. A helium plasma is generated by a microwave discharge in a He buffer gas and downstream, a small amount of argon gas is injected to destroy any helium metastables. A very small amount of neutral PAH molecules is added to the afterglow plasma by evaporation from a plate coated with the PAH to be studied. PAH ions are then produced by photoionization of the parent molecule using a pulsed UV laser (157 nm). The laser beam is oriented along the flow tube and so a constant spatial concentration of photoions is obtained. The electron concentration along the flow tube is measured by means of a movable Langmuir probe. Ion concentration decay in time is measured at a fixed position using a quadrupole mass spectrometer which is triggered by the laser pulse. The recombination of anthracene and pyrene cations has been studied using this technique and we have found a recombination rate of (2.4 +/- 0.8) x 10(-6) cm(3) s(-1) for anthracene and (4.1 +/- 1.2) x 10(-6) cm(3) s(-1) for pyrene.

C3H3+ Isomers: Temperature Dependencies of Production in the H3+ Reaction with Allene and Loss by Dissociative Recombination with Electrons

A technique has been developed to simultaneously determine recombination rate coefficients, alpha e, and initial concentrations of ion types that coexist in a flowing afterglow plasma. This was tested using the H3(+) + allene reaction in which two different C3H3+ isomers are produced. Use of an electrostatic Langmuir probe enabled the C3H3+ isomer branching ratios for propargyl and cyclic C3H3+ from this allene reaction and their alpha e to be determined over the temperature range 172-489 K. The study showed that the cyclic C3H3+ to propargyl C3H3+ branching ratios from the allene reaction varied from 50/50 at 172 K to 18/82 at 489 K. Over this temperature range, the alpha e for both isomers change only slightly. The room temperature alpha e values for propargyl and cyclic C3H3+ are (1.15 +/- 0.2) x 10(-7) and (8.00 +/- 0.1) x 10(-7) cm3/s, respectively. The data are discussed relative to current theories and in relation to fuel-rich flame chemistry, interstellar molecular synthesis, and modeling of Titan's atmosphere.

Yield of electronically excited N2 molecules from the dissociative recombination of N2H+ with e-.

Quantitative spectroscopic observations of the N2 first positive band system (N2(B 3Pig-A 3Sigmau+))/electron in a recombining N2H+ flowing-afterglow plasma indicate that a substantial fraction of the product N2 molecules are formed in one or more of the low-lying triplet states, B 3Pig, A 3Sigmau+, and W 3Deltau. The total measured N2(B-A) emission intensity from N2(B,v' > or = 1) is equivalent to a yield of (19 +/- 8)%. The effect of rapid collision-induced transitions between states of the triplet manifold is discussed..

Flowing-afterglow measurements of collisional radiative recombination of argon ions

Abstract Collisional-radiative recombination (CRR) of argon ions has been studied in the upstream region of a flowing-afterglow plasma. It is found that the recombination rate is accurately described by the CRR theory. In addition, the yield of metastable Ar( 3 P 2 ) and Ar( 3 P 0 ) atoms was determined by spectroscopic methods to be ∼35%, Ar( 3 P 2 ) being approximately six times more abundant than Ar( 3 P 0 ). The argon line spectrum produced by CRR consists largely of the near-infrared lines, belonging to 4p–4s and 4p ′ –4s ′ transitions.

I–V characteristics of the Langmuir probe in flowing afterglow plasmas

The specific features of the probe I–V characteristics in flowing-afterglow plasmas are studied experimentally and in theory. As it was found at a probe potential equal to the plasma one, V=0, an electron concentration in a probe vicinity (and a probe current) is decreased due to a predominant outflow of the electrons into an electrical circuit of the probe from the probe vicinity. The expression allowing one to reconstruct the undisturbed-by-probe electron concentration from only experimental data is derived. The reconstructed values of the electron concentration enable one to find from the experiments the semiempiric expressions allowing to describe quantitatively the behavior of the probe I–V characteristics at the electron-attracting as well as at the ion-attracting potential, respectively. The expressions found (both for electron-attracting and ion-attracting potential) include the “separating length,” which merely is the Langmuir length with a factor equal to the square root of the electron mass over the ion mass ratio for two-component plasma. The intermediate part of the probe I–V characteristics is discovered for probes operating with afterglow plasmas. This intermediate part is described in terms of the experimental parameter L0 having a dimension of the length (presumably electron-orbital length). The value of the parameter L0 does not depend on plasma parameters to within the ranges of plasma parameter variations for experimentally investigated plasmas as it was found. The experiments were performed with two cylindrical probes of 10 and 25 μm diam and ∼3 mm lengths in the experimentally investigated ranges of the afterglow plasma parameters: 105 cm−3<n<1010 cm−3, 0.017 eV<kT<0.3 eV, and 0.5 Torr⩽p⩽2 Torr of He gas pressure (corresponding to 950 μm⩾λTe⩾240 μm electron mean-free path) with (or without) Ar addition.

Enhanced formation of negative ions by electron attachment to highly excited molecules in a flowing afterglow plasma

Preliminary evidence for efficient negative-ion formation using a plasma mixing scheme was reported in a recent letter [L. A. Pinnaduwage, W. Ding, and D. L. McCorkle, Appl. Phys. Lett. 71, 3634 (1997)]. In the present article we confirm the negative ion formation using a probe-assisted photodetachment technique and estimate rate constants for electron attachment to electronically excited CH4 and NO in a flowing afterglow plasma. It is shown that enhanced electron attachment to molecules in highly excited states populated via excitation transfer from rare gas metastables is responsible for the observed negative ion formation. Implications for plasma processing and plasma remediation discharges are also discussed.

Electron-ion recombination rate coefficient measurements in a flowing afterglow plasma

The flowing-afterglow technique in conjunction with computer modelling of the flowing plasma has been used to determine accurate dissociative-recombination rate coefficients α for the ions O 2 +, HCO +, CH 5 +, C 2H 5 +, H 3O +, CO 2 +, HCO 2 +, HN 2O +, and N 2O + at 295 K. We find that the simple form of data analysis that was employed in earlier experiments was adequate and we largely confim earlier results. In the case of HCO + ions, published coefficients range from 1.1 × 10 −7 to 2.8 × 10 −7 cm 3/s, while our measurements give a value of 1.9 × 10 −7 cm 3/ s.

Recombination of H 3+ and D 3+ ions in a flowing afterglow plasma

The analysis of flowing afterglow plasmas containing H 3 + or D 3 + ions indicates that the de-ionization of such plasmas does not occur by simple dissociative recombination of ions with electrons. An alternative model of de-ionization is proposed in which electrons are captured into H 3 ∗∗ autoionizing Rydberg states that are stabilized by collisional mixing of the Rydberg molecules' angular momenta. The proposed mechanism would enable de-ionization to occur without the need for dissociative recombination by the mechanism of potential-surface crossings.

Langmuir Probe Determination of Charged Particle Number Density in a Flowing Afterglow Plasma

AbstractExperimental study of the electron and ion density determined by Langmuir probe is presented. The probe data have been obtained on FALP apparatus for electron (and ion) number densities ranging from 1012 m−3 to 1016 m−3, at electron temperatures equal to room temperature and also roughly ten times higher and at neutral pressures from 60 Pa to 150 Pa.The number densities of electrons for all measured probe data have been deduced from the electron current to the probe at the space potential and also by means of OML method from the electron current measured for positive probe with respect to the space potential. In the paper it is shown that in the flowing afterglow plasma the values of the electron density determined according both methods are in a good agreement and it is possible to take them as the reference number density for comparison with apparent ion densities obtained from the ion current.The number densities obtained from the ion currents in the ion accelerating region (negative probe with respect to the space potential) according to different applicable probe theories have been compared with reference value. It is found that the best agreement between the number densities evaluated from the electron current and from the ion current gives the theory of ion collection by Tichý et al. [16], which takes into account the collisions of ions with neutrals in the space charge sheath around the probe provided that the conditions correspond to the OML regime.

Langmuir-probe measurements in flowing-afterglow plasmas.

The validity of the orbital-motion theory for cylindrical Langmuir probes immersed in flowing-afterglow plasmas is investigated experimentally. It is found that the probe currents scale linearly with probe area only for electron-collecting but not for ion-collecting probes. In general, no agreement is found between the ion and electron densities derived from the probe currents. Measurements in recombining plasmas support the conclusion that only the electron densities derived from probe measurements can be trusted to be of acceptable accuracy. This paper also includes a brief derivation of the orbital-motion theory, a discussion of perturbations of the plasma by the probe current, and the interpretation of plasma velocities obtained from probe measurements.

FALP studies of the dissociative recombination coefficients for O 2+ and NO + within the electron temperature range 300–2000 K

Measurements are described of the dissociative recombination coefficients α of O 2 + and NO + at electron temperatures. T e within the range 300 K ⩽ T e⩽2000 K. These measurements have been achieved using the upgraded flowing afterglow—Langmuir probe (FALP) technique by which it is now possible to rapidly determine the T e (and also the electron energy distribution function) and the electron number density n e at any point along the axis of the flowing afterglow plasma. The elevated T e (above the carrier gas temperature T g) are readily achievable in argon carrier gas but not in helium carrier gas in which T e remains equal to T g. The α for both O 2 + and NO + are seen to vary with T e in power-law fashions, i.e. α(O 2 +) ≈ T e −0.65 and α(NO +)≈ T e −0.85 over the range of T e investigated, being the same as the power-law variations observed for isothermal conditions (i.e. T g= T e= T i, the ion temperature) in our previous FALP study within the range 200K⩽ T g⩽600 K. The implication of these results is that T e (not T i) primarily determines α(O 2 +) and α(NO +) in this temperature regime, in accordance with theoretical expectations. The ranges of T g and T e over which these α have now been determined are precisely those required for ionospheric deionization studies. The results obtained are compared with previous measurements, including some ionospheric in-situ measurements of α(NO +) as a function of T e, and, in general, the agreement between the previous and the present measurements is good.

Dependence of the Radial Profiles of the Plasma Particles Densities on the Afterglow Time in the Flowing Afterglow Plasma

AbstractThe simultaneous radial and axial decay in the flowing afterglow plasma has been investigated for metastable atoms, atomic and molecular ions and electrons. The exact solution of this problem leads to a set of differential equations. The stability of the numerical solution of these equations is not clear. The aim of the present paper is to simplify the mathematical description of the problem and to solve it on the basis of experimental results obtained in the argon flowing afterglow initiated by a microwave discharge.

A Probe Method for Determination of time Evolution of Metastable Atoms Density in a Flowing Afterglow Plasma

AbstractThe Langmuir Probe Measurement of the electron density and electron temperature has been made in the argon flowing afterglow plasma. Furthermore the metastable and charged particle densities evolution in the afterglow plasma are investigated by means of a numerical solution of the set of balance equations. From the fitting of the results of these calculations to the experimental data obtained from Langmuir probe measurements it is possible to determine the evolution of the metastable atoms density in the flowing afterglow plasma.

The Application of Langmuir Probes to the Measurements in Flowing Afterglow Plasma

AbstractLangmuir probe measurements in argon flowing afterglow are presented together with probe data analyses. The charged particles density is evaluated from the electron probe current at the space potential and from the saturated positive ion current. The latter is analysed using the method by Zakrzewski and Kopiczynski [15, 16] and both results are compared against each other. A satisfactory agreement almost within the estimated experimental error range suggests that the method [15, 16] brings a reasonable description of a positive ion collection by a Langmuir probe.

A contribution to the study of the influence of metastables in the flowing afterglow plasma

The paper describes an experimental apparatus based on the flowing afterglow technique which enables the study of the role of metastable atoms of the carrier gas by means of titration of the reactant vapour of gas into the flow tube. A study of the behaviour of the electron and ion density and electron distribution of Ar* metastables (Ar as the carrier gas) using Penning ionization of the ethyl alcohol (CH3CH2OH) (titration vapour) has been made by means of a described experimental set-up. The method of estimation of the metastables concentration in the decaying flowing afterglow plasma by Langmuir probe technique with use of titrant technique is also studied.

The electron distribution function in flowing afterglow argon plasma with the D.C. discharge plasma source

The measurements presented in this paper show that in our experimental conditions, in the flowing afterglow plasma generated by the D.C. plasma source the electron distribution function is not Maxwellian in the whole energetic region. In our experimental conditions it is not possible to use the electron temperature in the sense of the Maxwellian temperature.

On the emission of radiation from the neutralisation reaction NO++NO2-

A spectroscopic study has been made of the ion-ion neutralisation reaction NO++NO2- to neutral products in the wavelength range 180-600 nm. This was accomplished by observing the radiation emitted from a flowing afterglow plasma in which the dominant loss process for ionisation was the above neutralisation reaction. The only significant radiation observed was identified as the gamma bands of NO and it is deduced that the neutralisation reaction proceeds via a long-range electron transfer from the NO2- into the A 2 Sigma + state of NO, although population of the C 2 Pi and the D 2 Sigma + states cannot be ruled out.