Chemi-ionization Reactions Research Articles

On mixing of 2p53s excited neon atoms in highly non-equilibrium plasma

The experimental study of highly non-equilibrium neon plasma has been carried out when the 2p53s excited state atom density essentially exceeds the electron concentration. The initial electron density Neo was between 109 and 1010 cm-3, the density of the ground-state atoms N was within the limits 1017-1018 cm-3 and the reduced electric field E/N varied between 5 and 25 Td. The plasma was created as a result of an interaction between the stationary background and the pulsed electric fields. The excited atom and electron density growth induced by the stepwise electric field variations were found to be of an explosive character due to the accumulation of atoms in the lowest-lying excited states. The binary chemi-ionization reactions of the excited atoms were shown to play the principal role in the kinetics of the excited atoms and electrons. Due to the generation of fast electrons during the chemi-ionization processes, the electron energy distribution function was found to be a highly non-equilibrium one.

A study of some gas-phase lanthanide plus oxidant chemiionization reactions with chemielectron spectroscopy

An investigation has been made of some gas-phase lanthanide plus oxidant chemiionization reactions with electron spectroscopy. The oxidants used were O 2(X 3Σ g −), O 2(a 1Δ g) and O( 3P) and the lanthanide metals (Ln) studied were Pr, Nd, Sm, Eu and Gd. In the case of europium, no ions or electrons were observed for the Eu+O 2(X 3Σ g −), Eu+O 2(a 1Δ g) and Eu+O( 3P) reactions. For the other four lanthanides, studies of the equivalent metal oxidation reactions showed that when each metal is reacted with discharged oxygen, the dominant chemiionization channel is the associative ionization reaction Ln+O( 3P) → LnO ++e −. In each case, this conclusion was supported by comparison of available standard enthalpies for possible chemiionization reactions with the observed high kinetic energy offset of the observed chemielectron band, as well as by mass analysis of the ions produced. Results of earlier studies made by electron spectroscopy of the reactions of Ce and La with O 2(X 3Σ g −), O 2(a 1Δ g) and O( 3P) are discussed in the light of results obtained in this work.

Chemi-ionization produced by the catalytic combustion of a hydrocarbon

A mixture of 1000 p.p.m. propane and air was found to produce gas-phase ions when passed over a platinum filament, heated to ∼ 700°C or more. The ions were detected by direct sampling into a mass spectrometer. No negative ions were observed-only positively charged ones. It appears that these ions originate from chemi-ionization reactions occurring heterogeneously on the surface of the platinum. However, alkali metal impurities in the platinum contribute to the observed mass spectrum of positive ions. The observed spectra contain the relatively stable ions Na +, K +, and H 3O + and their hydrates. In fact, the original chemi-ions (possibly CHO +) disappear rapidly and form the more stable ions observed. These studies confirm the usefulness of heated platinum wires for sensing the presence of hydrocarbon gases in air.

Radiation spectroscopy of non-stationary plasma at high densities of excited noble-gas atoms

Theoretical and experimental study of the ionization-recombination relaxation in the pulsed discharge plasma has been carried out at high density of the excited noble-gas atoms. The background electron density Neb was between 109 and 1010 cm-3, the density of the neutral atoms Na was within the limits 1017-1018 cm-3, and the reduced electric field E/Na varied between 5 and 25 Td. The excited atom and electron density growth induced by the stepwise electric field variations was found to be of an explosive character due to the accumulation of atoms in the lowest electronically excited states. When the excited atom density exceeds the electron concentration essentially, the binary chemi-ionization reactions of the excited atoms were shown to play a principal role in the kinetics of the excited atoms and electrons. The theoretical description was performed in comparison with the experimental study of the kinetics of the excited atoms and electrons. Approximate analytical expressions have been obtained for the de-excitation development. The theoretical model allows calculation of the temporal dependence of the emission spectra and the determination of the effective rate coefficients of the direct excitation by electron impact from the ground state to the np5(n+1)s levels. The effective rate coefficients of the chemi-ionization processes for binary collisions between excited noble gas atoms have also been obtained. Calculation and experiment are in good agreement.

Dynamics of barium-bromine chemiionization reactions

The formation of BaBr{sup +} ions in reaction of Ba({sup 1}S{sub 0},{sup 1}P{sub 1}) with Br{sub 2} was studied as a function of laboratory scattering angle and product translational energy in a crossed-beam experiment. The contour map of BaBr{sup +} flux obtained for the ground-state reaction at 1.1-eV collision energy showed a backscattered angular distribution (relative to the barium beam) with a substantial fraction of the available energy appearing in translation. Laser excitation strongly inhibited this channel at 1.1 eV. These experimental observations suggest that for the chemiion reaction head-on, collinear collisions and proximal crossings of the potential energy surfaces are necessary to preclude escape into the dominant neutral pathways. At 1.6-eV collision energy potential energy surfaces are necessary to preclude escape into the dominant neutral pathways. At 1.6-eV collision energy a new laser-dependent source of BaBr{sup +} appeared. This laser-enhanced BaBr{sup +} showed a laboratory angular distribution substantially narrower than for the ground-state reaction, indicating a smaller translational energy release. The angular distribution was {approximately}70% backscattered and displayed a clear dip at the center of mass. This new BaBr{sup +}, produced from electronically excited barium at higher translational energy, is ascribed to secondary collisions of BaBr+Br initially formed inmore » low impact parameter collisions. Reaction of electronically excited barium with Br{sub 2} also yielded an associative ionization product BaBr{sub 2}{sup +} at both collision energies studied, with a cross section about 1/100th that of the chemiion channel.« less

Crossed-beam study of the dynamics of chemiionization reactions

Crossed-beam study of the dynamics of chemiionization reactions

Absolute calibration of a retarding-potential Mott polarimeter

A novel technique for absolute calibration of a Mott polarimeter is described that makes use of electrons of accurately known polarization obtained through chemiionization reactions involving spin polarized He(2 3S) metastable atoms. This technique has been used to calibrate a compact retarding-potential Mott polarimeter and values of the effective asymmetry (Sherman) function Seff are presented for both gold and thorium target foils under a variety of operating conditions.

Improved source of polarized electrons based on a flowing helium afterglow

The performance of the Rice source of spin polarized electrons, which is based on an optically pumped flowing helium afterglow, has been substantially improved. He(23S) metastable atoms contained in the afterglow are optically pumped using 1.08 μm 23S1↔23P1 radiation from an LNA laser. Spin conservation in subsequent chemi-ionization reactions with CO2 results in the production of free polarized electrons that are extracted from the afterglow. At low currents, ≲1 μA, polarizations of 80%–90% are achieved. This decreases to ∼75% at 10 μA and to ∼50% near 100 μA. The polarization can be simply reversed (P→−P). The energy spread in the extracted beam is <0.4 eV, and the beam emittance is <4 mrad cm−1 at 270 eV. This source is suitable for use in a wide variety of applications, and is particularly attractive for use with the new generation of high-duty factor electron accelerators that are currently being developed.

Evidence for two independent chemi-ionization processes in the O+C 2H 2 reaction

Gases which are known to react rapidly with CH(X 2Π) are added to low pressure mixtures of oxygen atoms and acetylene while monitoring chemi-ion formation. Addition of CH 4 and N 2O, which do not react with oxygen atoms at room temperature, rapidly quench part of the chemi-ion formation, but further additions do not quench it completely. The data can be fit to a simple kinetic model that assumes two independent chemi-ionization reactions. Mass analysis shows that HCO + is the primary ion for both processes. We propose that the non-quenchable chemi-ionization is caused by the reaction of oxygen atoms with the metastable CH(a 4Σ).

Chemiionization and ion-molecule reactions in fuel-rich acetylene flames

The objective of this study is to model the appearance and behavior of combustion-generated ions in rich flames of acetylene. A one-dimensional, laminar, flat flame of acetylene was developed that combines reactions for oxidation, pyrolysis, chemiionization, proton transfer, ion-molecule reactions, and charge recombination relevant to rich acetylene combustion. Four possible chemiionization reactions, CH+O⇄ CHO ++ e − CH ∗+ O⇄ CHO ++ e − CH ∗+ C 2 H 2⇄C 3H 3 ++ e − C 2+CH 3⇄ C 3 H 3 ++ e − are included in the model. Ions other than CHO + and C 3H 3 + are assumed to form by proton transfer or by a series of ion-molecute reactions. The proposed model predicts total ion concentrations reasonably well over a wide range of stoichiometries. Total ion concentrations are very sensitive to the rate of the CH ∗ + O chemiionization reaction. The dominant ion, C 3H 3 +, is found to be produced from CHO + by ion-molecule reactions rather than by direct chemiionization. Computations show that only a limited number of ionic reactions are equilibrated in the flames. The ratio of large ions to small ions is predicted to increase sharply as the critical equivalence ratio for soot formation is approached.

Chemielectron spectroscopy: A study of the gas-phase reactions of uranium with O 2 and N 2O

The gas-phase chemiionization reactions of uranium with oxygen and uranium with nitrous oxide have been studied using electron spectroscopy. Both reactions show one band in the electron spectrum. These bands have different maxima and shapes which are rationalised using schematic potential energy surfaces.

Molecular-beam study of chemiionization reactions of antimony pentafluoride

Molecular-beam study of chemiionization reactions of antimony pentafluoride

Ions and soot in flames

An appreciation of the work of Sir Morris Sugden is given. In this study the positive and negative ions in five oxyacetylene flames were observed mass spectrometrically. The flames ranged from one which was fairly fuelrich (C/O = 0.80) to ones with a feather (including sooting flames) with C/O = 1.09. A great variety of ions was found. The positive ions could be divided into four groups: (i) those containing oxygen; (ii) polyacetylenic ions, i.e. CxH+3(x= 3–13); (iii) more hydrogenated, but aliphatic CxH+y ions (with y > 3); and (iv) aromatic CxH+y ions. It is concluded that ions in these very fuel-rich flames originate from CH*+C2H2→ C3H+3+ e– as well as CH + O → CHO++ e–. Many of the positive ions are subsequently formed in effect by protonation of those molecules and radicals which are either intermediates or products of combustion. There is evidence that C2H2 in these flames forms a wide variety of hydrocarbon radicals and polyacetylene and aromatic molecules. As for oxygen-containing species, the protonated versions of ketene, hydroxyketene, ethanol, formic acid, carbon monoxide and water appear, particularly in the less fuel-rich flames.The observed negative ions fall into three groups: (i) H3CO–3 and H3CO–4, which are the only ones present throughout a flame; (ii) hydrocarbon ones which are ‘relatives’ of the positive ions in that they appear to derive from the same uncharged radicals and molecules; (iii) short-lived oxygen-containing ions, e.g. CHO–2, O–2, C2HO–(deprotonated ketene), HO–2, OH– and O–. Their reactions are discussed along with those of the free electrons, which are also present in these flames.An additional ion-producing mechanism is found in the richest flame just before soot particles visibly appear in it. This seems to be a chemiionization reaction between an aromatic molecule and a hydrocarbon radical, an illustrative example being: C16H10+ C6H → C22H+11+ e–. Soot production can then proceed by rapid growth processes involving C2H2 in ion–molecule reactions of the type: CxH+y+ C2H2→ Cx+2H+y+ H2. The resulting large clusters grow to particles or semi-liquid droplets which either coagulate or coalesce to yield soot. In turn these particles are observed to ionize thermally. Thus the production of soot appears to derive, at least partly, from fairly small aromatic molecules. Their production from C2H2etc. in these flames is discussed, as also are the structures of some of these flame ions.

Properties of gas-phase ions. Information to be obtained from photoelectron spectroscopy of unstable molecules

The information to be obtained from the study of unstable molecules in the vapour phase by photoelectron spectroscopy is reviewed, using examples drawn from high-temperature pyrolysis experiments and rapid atom-molecule reaction. An example is given of a chemi-ionization reaction studied by electron spectroscopy, and the information to be obtained from such studies is outlined.

Chemi-ionization reactions of barium with chlorine clusters: Observation of Cl−m and BaCl+m products (m=1–15)

Chemi-ionization reactions from the collision of Ba with (Cl2) clusters are reported. The scattering yields BaCl+m as well as Cl−m cluster ions. Relative integral cross sections for products with m even or odd depend differently on the collision energy. Over the collision energy range studied of a few eV, channels with odd products predominate, and at higher collision energy constant fractions characterize the observed product pattern. Aspects of the collision mechanism involved are discussed. The analysis suggests these reactions to start within the subsystem Ba+Cl2. The corresponding relative cross section has been determined for Ec.m. ≤2.7 eV.

Chemi-ionization reactions of state-selected electronically excited Ar(3P0) and Ar(3P2) atoms

Using a chemical method to separate the spin-orbit Ar(3P0) and Ar(3P2) metastable states in a discharge-flow system, branching fractions, f0 and f2, for ionization of several molecular reagents by these species have been measured. Experiments have been performed at room temperature and at 220 K. The large differences between f0 and f2 observed for CCl4, CF3Br, HBr, and DBr are ascribed to energy effects and it is concluded that the yield of chemi-ionization increases sharply as the electronic energy of the excited atom rises above the ionization potential of the molecule. This contrasts with the much smaller effect of reagent kinetic energy on the chemi-ionization yield. Branching fractions for some reaction channels leading to emission of light are also reported.

Direct observations of chemi-ionization in hydrocarbon flames enhanced by laser excited CH*(A 2 Δ) and CH*(B 2Σ −)

An experimental method based on the detection of the enhancement of chemi-ionization following the pulsed laser excitation of CH*(A 2Δ, B 2Σ −) radicals is described. This approach provides conclusive and direct observations of chemi-ionization reactions involving CH*(A 2Δ, B 2Σ − radicals. The results suggest that the rate constants for the reactions CH*(A 2Δ, B 2Σ −) + O − HCO + + e − are approximately 2000 times faster at 2100 K than the previously measured rate constant for the reaction CH(X 2Π) + O → HCO + + e − which is believed to be a primary source of chemi-ionization in all hydrocarbon flames.

Dynamics of the chemiionization reaction of antimony pentafluoride

Mesure des distributions angulaires de produits et d'energie pour SbF 5 +C 6 H 5 COCl→SbF 5 Cl − +C 6 H 5 CO + et pour Sb 2 F 10 +C 6 H 5 CH 2 Cl→SbF 6 − +C 7 H 7 + +SbF 4 Cl

Chemi-ionization reactions of electronically excited lithium

Chemi-ionization reactions of atomic lithium in the 4 2S1/2 state have been observed with NO2, Cl2, and F2 collision partners in a crossed-beam system. The reaction products have been analyzed with a mass filter to be Li++NO2−, LiCl++Cl−, and Li++F+F−, with reaction cross sections of 0.02, 0.2, and 2×10−16 cm2, respectively.

A flow tube investigation of the temperature dependence of the chemi-ionization reaction between antimony pentafluoride and benzyl chloride

The temperature dependence of the reaction between antimony penfluoride and benzyl chloride has been investigated using flow tube methods. Under the assumption that a single species of (SbF5)n is involved in the reaction, an activation energy of 22±2 kcal mol−1 is obtained. The bimolecular rate constant is estimated to be 10−16 cd molecule−1 sec−1 at 500 °K, and the reaction section 10−5 Å2. (AIP)