Flowing-afterglow Langmuir-probe Apparatus Research Articles

Excited-State N Atoms Transform Aromatic Hydrocarbons into N-Heterocycles in Low-Temperature Plasmas.

The direct formation of N-heterocycles from aromatic hydrocarbons has been observed in nitrogen-based low-temperature plasmas; the mechanism of this unusual nitrogen-fixation reaction is the topic of this paper. We used homologous aromatic compounds to study their reaction with reactive nitrogen species (RNS) in a dielectric barrier discharge ionization (DBDI) source. Toluene (C7H8) served as a model compound to study the reaction in detail, which leads to the formation of two major products at "high" plasma voltage: a nitrogen-replacement product yielding protonated methylpyridine (C6H8N+) and a protonated nitrogen-addition (C7H8N+) product. We complemented those studies by a series of experiments probing the potential mechanism. Using a series of selected-ion flow tube experiments, we found that N+, N2+, and N4+ react with toluene to form a small abundance of the N-addition product, while N(4S) reacted with toluene cations to form a fragment ion. We created a model for the RNS in the plasma using variable electron and neutral density attachment mass spectrometry in a flowing afterglow Langmuir probe apparatus. These experiments suggested that excited-state nitrogen atoms could be responsible for the N-replacement product. Density functional theory calculations confirmed that the reaction of excited-state nitrogen N(2P) and N(2D) with toluene ions can directly form protonated methylpyridine, ejecting a carbon atom from the aromatic ring. N(2P) is responsible for this reaction in our DBDI source as it has a sufficient lifetime in the plasma and was detected by optical emission spectroscopy measurements, showing an increasing intensity of N(2P) with increasing voltage.

Reactions of C+ + Cl-, Br-, and I--A comparison of theory and experiment.

Rate constants for the reactions of C+ + Cl-, Br-, and I- were measured at 300 K using the variable electron and neutral density electron attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. Upper bounds of <10-8 cm3 s-1 were found for the reaction of C+ with Br- and I-, and a rate constant of 4.2 ± 1.1 × 10-9 cm3 s-1 was measured for the reaction with Cl-. The C+ + Cl- mutual neutralization reaction was studied theoretically from first principles, and a rate constant of 3.9 × 10-10 cm3 s-1, an order of magnitude smaller than experiment, was obtained with spin-orbit interactions included using a semiempirical model. The discrepancy between the measured and calculated rate constants could be explained by the fact that in the experiment, the total loss of C+ ions was measured, while the theoretical treatment did not include the associative ionization channel. The charge transfer was found to take place at small internuclear distances, and the spin-orbit interaction was found to have a minor effect on the rate constant.

Mutual neutralization of H+ and D+ with the atomic halide anions Cl−,Br−, and I−

Mutual neutralization (MN) rate constants kMN for the reactions of H+ and D+ with the atomic halide anions Cl-, Br-, and I- were measured using the variable electron and neutral density attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. At 300 K, the rate constants for each reaction studied are on the order of 10-8 cm3 s-1. A trend for the rate constants of the systems in this work, kMNCl-<kMNBr-<kMN(I-), is consistent with prior studies of rare gas cation with atomic halide anion MN. A recent theoretical study involving ab initio quantum mechanical treatment of the H++Cl- and D++Cl- reactions reported rate constants significantly lower than the rates reported here. A previously proposed empirical model that predicts atom-atom kMN as a simple function of the total reaction exothermicity shows good agreement with the newly measured rate constants.

Time-of-flight detection coupled to a flowing afterglow: Improvements and characterization

An orthogonally accelerated time-of-flight mass spectrometer has been added to a flowing afterglow–Langmuir probe apparatus, coupled by a rectilinear ion guide. The TOF has improved the signal-to-noise (S/N) ratio by about an order of magnitude over a quadrupole mass filter/electron multiplier detection typical for flow tube instruments of this type and used previously. The improved S/N reduces our data acquisition time commensurately. Additionally, mass discrimination is low enough that it is minimal for ions heavier than ∼10amu. The lack of mass discrimination not only reduces experiment time but also leads to greater accuracy in the measurement of reaction rate coefficients. Finally, simultaneous detection of both heavy and light ions is improved. H+ can now be detected and WF5+ isotopes can be separated without reducing sensitivity. We have found that discrimination against low-mass ions can occur when certain electron-attaching reactants are used (e.g. SF6), and possibly due to radicals formed after certain other reactants attach electrons. Evidence points to adsorption of these species on surfaces of the ion guide immediately behind the flowing afterglow sampling orifice, which separates the flow tube and TOF region. The adsorption follows a Langmuir isotherm profile and is greatly reduced by heating the gas in the flow tube.

Electron attachment to 14 halogenated alkenes and alkanes, 300-600 K

Thermal electron attachment to 14 alkenes and alkanes with bromine, fluorine, and iodine substituents has been studied over the temperature range 300-600 K using a flowing-afterglow Langmuir-probe apparatus. Rate coefficients and anion products are reported, most for the first time. Among these were 3 isomers of C(3)F(5)Br and the 2 isomers of C(3)F(7)I. Four dibromide compounds were studied, all of which yield Br(2)(-) product in addition to Br(-) product. The results are analyzed using a statistical kinetic modeling approach, which is able to reproduce both attachment rate coefficients and product branching ratios within experimental uncertainty. The kinetic modeling indicates that factor of 2 differences in attachment rate coefficients to the isomeric species can be explained by subtle variations in the potential surfaces.

Electron Attachment to C7F14, Thermal Detachment from C7F14–, the Electron Affinity of C7F14, and Neutralization of C7F14– by Ar+

Rate coefficients and branching fractions have been measured for electron attachment to perfluoromethylcyclohexane, C(7)F(14), along with thermal detachment rate coefficients for C(7)F(14)(-), from 300 to 630 K, using a flowing-afterglow Langmuir-probe apparatus. The attachment rate coefficient at room temperature is 4.5 ± 1.2 × 10(-8) cm(3) s(-1) and increases with temperature at a rate described by an activation energy of 50 ± 25 meV. Thermal electron detachment is negligible at room temperature, but measurable at 600 K and above, reaching 2300 ± 1300 s(-1) at 630 K. Analysis of the attachment-detachment equilibrium yields EA(C(7)F(14)) = 1.02 ± 0.06 eV, in agreement with the literature value while more than halving the uncertainty. Implications of the measurement for the electron affinity of SF(6) are discussed. The dominant product of electron attachment is the parent anion, but C(6)F(11)(-) and C(7)F(13)(-) are also observed at very low levels (<0.1%) at room temperature and increase in importance as the temperature is increased, reaching ~10% each at 630 K. In the course of this work we have also measured rate coefficients for the neutralization of C(7)F(14)(-) by Ar(+) at 300, 400, and 500 K: 4.8, 3.5, and 3.1 × 10(-8) cm(3) s(-1), respectively, with uncertainties of ±5 × 10(-9) cm(3) s(-1).

Electron Attachment to Fe(CO)n (n = 0–5)

The rate constants of thermal electron attachment at 300 and 400 K to Fe(CO)(n) (n = 0-5) have been measured using a flowing afterglow Langmuir probe apparatus. The stable species Fe(CO)(5) was studied using the traditional method of monitoring electron depletion as a function of reaction time, and the remaining short-lived species were studied using the variable electron and neutral density attachment mass spectrometry (VENDAMS) technique. Attachment to Fe(CO)(5) is purely dissociative and about 20% efficient with a rate constant of (7.9 ± 1.4) × 10(-8) cm(3) s(-1) at 300 K and (8.8 ± 2) × 10(-8) cm(3) s(-1) at 400 K. The attachment rate constants decrease significantly as each CO ligand is removed, with Fe(CO)(n) (n = 4 to 1) attaching with efficiencies on the order of 10%, 1%, 0.1%, and 0.01% respectively. Under the conditions here, attachment to Fe(CO)(4) and Fe(CO)(3) are likely entirely dissociative, whereas attachment to Fe(CO)(2) and Fe(CO) are almost entirely associative. A statistical kinetic modeling approach is used to explain the strong dependence of the attachment rate constant on the number of ligands present in the neutral species through a combination of increasing autodetachment rates and decreasing exothermicities to dissociative attachment. The VENDAMS data also define the 300 K mutual neutralization rate constant of Fe(CO)(4)(-) + Ar(+) to be (5.0 ± 0.8) × 10(-8) cm(3) s(-1) with an upper limit to branching fraction of 0.5 to yield Fe(CO)(4), indicating that significant fragmentation to smaller Fe(CO)(n) occurs.

Electron attachment to propargyl chloride, 305–540 K

Electron attachment to propargyl chloride (HC≡C-CH(2)Cl) was studied in a flowing-afterglow Langmuir-probe apparatus from 305 to 540 K. The sole ion product in this temperature range is Cl(-). Electron attachment is very inefficient, requiring correction for a competing process of electron recombination with molecular cations produced in reaction between Ar(+) and propargyl chloride and subsequent ion-molecule reactions. The electron attachment rate coefficient was measured to be 1.6×10(-10)cm(3) s(-1) at 305 K and increased to 1.1×10(-9)cm(3) s(-1) at 540 K.

Experimental and modeling study of thermal rate coefficients and cross sections for electron attachment to C60

Thermal electron attachment to C(60) has been studied by relative rate measurements in a flowing afterglow Langmuir probe apparatus. The rate coefficients of the attachment k(1) are shown to be close to 10(-6) cm(3) s(-1) with a small negative temperature coefficient. These results supersede measurements from the 1990s which led to much smaller values of k(1) with a large positive temperature coefficient suggesting an activation barrier. Theoretical modeling of k(1) in terms of generalized Vogt-Wannier capture theory shows that k(1) now looks more consistent with measurements of absolute attachment cross sections sigma(at) than before. The comparison of capture theory and experimental rate or cross section data leads to empirical correction factors, accounting for "intramolecular vibrational relaxation" or "electron-phonon coupling," which reduce k(1) below the capture results and which, on a partial wave-selected level, decrease with increasing electron energy.

Electron attachment to chlorine azide at 298 and 400 K

Electron attachment to chlorine azide (ClN(3)) was studied using a flowing-afterglow Langmuir-probe apparatus. Electron attachment rates were measured to be 3.5x10(-8) and 4.5x10(-8) cm(3) s(-1) at 298 and 400 K, respectively, with an estimated 35% absolute accuracy. Cl(-) was the sole ion product of the attachment reaction; weak ion signals were observed for other anions and attributed to impurities and secondary ion-molecule reactions. Assuming a relative uncertainty of +/-10% for these data, an activation energy for the attachment reaction may be given as 24+/-10 meV.

Electron Affinity of trans-2-C4F8 from Electron Attachment−Detachment Kinetics

Electron attachment and detachment kinetics of 2-C(4)F(8) were studied over the temperature range 298-487 K with a flowing-afterglow Langmuir-probe apparatus. Only parent anions were formed in the attachment process throughout this temperature range. At the highest temperatures, thermal electron detachment of the parent anions is important. Analysis of the 2-C(4)F(8) gas showed an 82/18 mixture of trans/cis isomers. The kinetic data at the higher temperatures were used to determine the electron affinity EA(trans-2-C(4)F(8)) = 0.79 +/- 0.06 eV after making some reasonable assumptions. The same quantity was calculated using the G3(MP2) compound method, yielding 0.74 eV. The kinetic data were not sufficient to establish a reliable value for EA(cis-2-C(4)F(8)), but G3(MP2) calculations give a value 0.017 eV greater than that for trans-2-C(4)F(8). MP2 and density functional theory were used to study the structural properties of the neutral and anion isomers.

Electron attachment to halomethanes at high temperature: CH2Cl2, CF2Cl2, CH3Cl, and CF3Cl attachment rate constants up to 1100 K

We have used a high-temperature flowing-afterglow Langmuir-probe apparatus to measure rate constants for electron attachment to halomethanes which attach electrons very inefficiently at room temperature, yielding Cl(-) ion product. We studied CH(2)Cl(2) (495-973 K), CF(2)Cl(2) (291-1105 K), and CF(3)Cl (524-1004 K) and include our recent measurement for CH(3)Cl (700-1100 K) in the discussion of the electron attachment results. The measured attachment rate constants show Arrhenius behavior in the temperature ranges examined, from which estimates of rate constants at 300 K may be made: CH(2)Cl(2) (1.8x10(-13) cm(3) s(-1)), CH(3)Cl (1.1x10(-17) cm(3) s(-1)), and CF(3)Cl (4.2x10(-14) cm(3) s(-1)), all of which are difficult to measure directly. In the case of CF(2)Cl(2), the room temperature rate constant was sufficiently large to be measured (1.6x10(-9) cm(3) s(-1)). The Arrhenius plots yield activation energies for the attachment reactions: 390+/-50 meV (CH(2)Cl(2)), 124+/-20 meV (CF(2)Cl(2)), 670+/-70 meV (CH(3)Cl), and 406+/-50 meV (CF(3)Cl). Comparisons are made with existing data where available. G3 calculations were carried out to obtain reaction energetics. They show that the parent anions of CH(2)Cl(2) CF(2)Cl(2), CH(3)Cl, and CF(3)Cl are stable, though CH(3)Cl(-) exists only as an electrostatically bound complex.

Electron attachment toCl2from 300 to 1100 K: Experiment and theory

Rate constants for dissociative electron attachment to ${\text{Cl}}_{2}$ have been measured from 300 to 1100 K in a high-temperature flowing-afterglow Langmuir-probe apparatus. $R$-matrix calculations have been carried out which compare well with the present measurements with possible deviation at the highest temperatures. The attachment rate constants do not show Arrhenius behavior. The temperature dependence of the calculated rate constants for successive vibrational levels provides insight as to this behavior. While the lowest vibrational level of ${\text{Cl}}_{2}$ dominates attachment at low temperatures, the rate constant is not flat with temperature because of the $p$-wave character of the attachment process. The non-Arrhenius behavior is due to a conflict between the increase in attachment cross section with vibrational level (temperature) and the decline in the cross section with electron energy above 50 meV.

Electron attachment to SF6 under well defined conditions: comparison of statistical modeling results to experiments

Experiments were carried out using a flowing-afterglow Langmuir-probe apparatus to measure rate constants for electron attachment to SF6 and thermal detachment from SF6−. In a recent series of papers, these results were combined with new and existing data on nondissociative and dissociative attachment to SF6 and compared to statistical modeling of the various processes involved in the stabilization of the ionic products of attachment. This paper gives a summary of those findings. The major conclusions are: (a) only the ground electronic state of SF6− needs to be invoked to explain available data; (b) the electron affinity of SF6 is higher than previously thought, namely, EA(SF6) = 1.20 (± 0.05) eV; (c) the endothermicity of the dissociative electron attachment reaction that yields SF5− is 0.41 eV (± 0.05) eV at 0 K; (d) combining these two numbers gives the bond energy D0o(F—SF5−) = 1.61 (± 0.05) eV.

Electron attachment to Ni(PF3)4 and Pt(PF3)4

An experimental study has been made of thermal electron attachment to the transition-metal trifluorophosphine complexes Ni(PF3)4 and Pt(PF3)4 using a flowing-afterglow Langmuir-probe apparatus. Both complexes are efficient at electron attachment, although the rate constants are somewhat less than collisional. The rate constant for electron attachment to Ni(PF3)4 is 1.9×10−7cm3s−1 at room temperature, about a factor of 2 less than collisional. The activation energy is 39±5meV for the attachment reaction. The rate constant for electron attachment to Pt(PF3)4 is 5.4×10−8cm3s−1 at room temperature, and the activation energy is 84±8meV. For both complexes, a PF3 ligand is lost on electron attachment, and only the M(PF3)3− ion is observed in the negative-ion mass spectrum. Density functional calculations were carried out on Ni(PF3)4 and various fragments in order to describe the thermochemistry of the attachment reaction.

Low-energy electron attachment to SF6. III. From thermal detachment to the electron affinity of SF6

The thermal attachment of electrons to SF(6) is measured in a flowing-afterglow Langmuir-probe apparatus monitoring electron concentrations versus axial position in the flow tube. Temperatures between 300 and 670 K and pressures of the bath gas He in the range of 0.3-9 Torr are employed. Monitoring the concentrations of SF(6)(-) and SF(5)(-), the latter of which does not detach electrons under the applied conditions, an onset of thermal detachment and dissociation of SF(6) at temperatures above about 530 K is observed. Analysis of the mechanism allows one to deduce thermal detachment rate coefficients. Thermal dissociation rate coefficients for the reaction SF(6)(-)-->SF(5)(-)+F can only be estimated by unimolecular rate theory based on the results from Part I and II of this series. Under the applied conditions they are found to be smaller than detachment rate coefficients. Combining thermal attachment and detachment rates in a third-law analysis, employing calculated vibrational frequencies of SF(6) and SF(6)(-), leads to the electron affinity (EA) of SF(6)(-). The new value of EA=1.20(+/-0.05) eV is significantly higher than previous recommendations which were based on less direct methods.

A new technique for studying ion–ion recombination in a flowing afterglow Langmuir probe apparatus: Ar + recombining with Cl 2−, CCl 2O −, Br 2−, SF 5− and SF 6−

We present a new technique for measuring ion–ion recombination rate constants in a flowing afterglow Langmuir probe (FALP) apparatus. The technique involves measuring the fractional negative ion product distribution following electron attachment versus the initial electron density when two or more products are formed. The concentration of reactant gas is kept low enough that the plasma retains its electron-Ar +, ambipolar diffusion character along the entire length of the flow tube. If only polyatomic anions are formed, accurate relative rates are obtained. When one of the species is atomic, absolute rates are also possible by doing a detailed model of the plasma kinetics. Here we present rate constants for Ar + recombining with Cl 2 − ((5.3 ± 1.6) × 10 −8 cm 3 s −1 at 302 K), Br 2 − ((3.9 ± 1.2) × 10 −8 cm 3 s −1 at 302 K), the phosgene negative ion CCl 2O − ((8.9 ± 2.7) × 10 −8 cm 3 s −1 at 302 K), and relative rate constants for Ar + + SF 6 − and SF 5 − (ratio 1.2 at 550 K, with an uncertainty of +0.3 and −0.1). The diatomic negative ions are found to recombine slower than the polyatomic ones, in agreement with earlier indications.

Electron attachment to MoF6, ReF6, and WF6; reaction of MoF6− with ReF6 and reaction of Ar+ with MoF6

Rate constants were measured for electron attachment to MoF(6), ReF(6), and WF(6) in 133 Pa of helium gas using a flowing-afterglow Langmuir-probe apparatus. The experiment is a thorny one because the molecules tend to form oxide impurities on feedline surfaces and because of thermal decomposition of MoF(6) on surfaces as the gas temperature is increased. The electron attachment rate constant for MoF(6) is (2.3+/-0.8)x10(-9) cm(3) s(-1) at 297 K; only MoF(6) (-) is formed in the temperature range of 297-385 K. The rate constant increases with temperature up to the point where decomposition becomes apparent. Electron attachment to ReF(6) occurs with a rate constant of (2.4+/-0.8)x10(-9) cm(3) s(-1) at 297 K; only ReF(6) (-) is produced. MoF(6) (-) reacts with ReF(6) to form ReF(6) (-) on essentially every collision, showing definitively that the electron affinity of ReF(6) is greater than that of MoF(6). A rate constant of (5.0+/-1.3)x10(-10) cm(3) s(-1) was measured for this ion-molecule reaction at 304 K. The reverse reaction is not observed. The reaction of Ar(+) with MoF(6) was found to produce MoF(5) (+)+F, with a rate constant of (1.8+/-0.5)x10(-9) cm(3) s(-1). WF(6) attaches electrons so slowly at room temperature that the attachment rate was below detection level (< or =10(-12) cm(3) s(-1)). By 552 K, the attachment rate constant reaches a value of (2+/-1)x10(-10) cm(3) s(-1).

Electron attachment and detachment, and the electron affinities of C5F5N and C5HF4N

Rate constants have been measured for electron attachment to C5F5N (297-433 K) and to 2, 3, 5, 6-C5HF4N (303 K) using a flowing-afterglow Langmuir-probe apparatus (at a He gas pressure of 133 Pa). In both cases only the parent anion was formed in the attachment process. The attachment rate constants measured at room temperature are 1.8 +/- 0.5 X 10(-7) and 7 +/- 3 X 10(-10) cm(-3) s(-1), respectively. Rate constants were also measured for thermal electron detachment from the parent anions of these molecules. For C5F5N- detachment is negligible at room temperature, but increases to 2530 +/- 890 s(-1) at 433 K. For 2, 3, 5, 6-C5HF4N-, the detachment rate at 303 K was 520 +/- 180 s(-1). The attachment/detachment equilibrium yielded experimental electron affinities EA(C5F5N)=0.70 +/- 0.05 eV and EA(2, 3, 5, 6-C5HF4N)=0.40 +/- 0.08 eV. Electronic structure calculations were carried out for these molecules and related C5HxF5-xN using density-functional theory and the G3(MP2)//B3LYP compound method. The EAs are found to decrease by 0.25 eV, on average, with each F substitution by H. The calculated EAs are in good agreement with the present experimental results.

Electron attachment and detachment: Electron affinities of isomers of trifluoromethylbenzonitrile

Rate constants for electron attachment to the three isomers of trifluoromethylbenzonitrile [(CF(3))(CN)C(6)H(4), or TFMBN] were measured over the temperature range of 303-463 K in a 133-Pa He buffer gas, using a flowing-afterglow Langmuir-probe apparatus. At 303 K, the measured attachment rate constants are 9.0 x 10(-8) (o-TFMBN), 5.5 x 10(-8) (m-TFMBN), and 8.9 x 10(-8) cm(3) s(-1) (p-TFMBN), estimated accurate to +/-25%. The attachment process formed only the parent anion in all three cases. Thermal electron detachment was observed for all three anion isomers, and rate constants for this reverse process were also measured. From the attachment and detachment results, the electron affinities of the three isomers of TFMBN were determined to be 0.70(o-TFMBN), 0.67(m-TFMBN), and 0.83 eV (p-TFMBN), all +/-0.05 eV. G3(MP2) [Gaussian-3 calculations with reduced Møller-Plesset orders (MP2)] calculations were carried out for the neutrals and anions. Electron affinities derived from these calculations are in good agreement with the experimental values.