Jordan C. Sawyer

O2 rotational temperature measurements in an atmospheric air microdischarge by radar resonance-enhanced multiphoton ionization

Nonintrusive spatially resolved rotational temperature measurements in an atmospheric air microdischarge are presented. The measurements were based on coherent microwave Rayleigh scattering (Radar) from resonance-enhanced multiphoton ionization of molecular oxygen. The open air DC microdischarge source operated in a stable “normal-glow” mode and pin-to-pin electrodes spaced 1.3 mm apart. The second harmonic of a tunable dye laser beam was focused between the two electrodes and scanned between 286 and 288 nm. Coherent microwave Rayleigh scattering was used to collect the two-photon rotational spectra of O2 at C3Π(v = 2)←X3Σ(v′ = 0) transitions. The Boltzmann plots from analyses of the O2 rotational lines determined local rotational temperatures at various axial locations between the electrodes. The molecular oxygen rotational temperature varied from ∼1150 K to ∼1350 K within the discharge area. The measurements had an accuracy of ∼±50 K.

Kinetics and product branching fractions of reactions between a cation and a radical: Ar(+) + CH3 and O2(+) + CH3.

A novel technique is described for the measurement of rate constants and product branching fractions of thermal reactions between cation and radical species. The technique is a variant of the variable electron and neutral density attachment mass spectrometry (VENDAMS) method, employing a flowing afterglow-Langmuir probe apparatus. A radical species is produced in situ via dissociative electron attachment to a neutral precursor; this allows for a quantitative derivation of the radical concentration and, as a result, a quantitative determination of rate constants. The technique is applied to the reactions of Ar(+) and O2(+) with CH3 at 300 K. The Ar(+) + CH3 reaction proceeds near the collisional rate constant of 1.1 × 10(-9) cm(3) s(-1) and has three product channels: → CH3(+) + Ar (k = 5 ± 2 × 10(-10) cm(3) s(-1)), → CH2(+) + H + Ar (k = 7 ± 2 × 10(-10) cm(3) s(-1)), → CH(+) + H2 + Ar (k = 5 ± 3 × 10(-11) cm(3) s(-1)). The O2(+) + CH3 reaction is also efficient, with direct charge transfer yielding CH3(+) as the primary product channel. Several results needed to support these measurements are reported, including the kinetics of Ar(+) and O2(+) with CH3I, electron attachment to CH3I, and mutual neutralization of CH3(+) and CH2(+) with I(-).

Reactivity from excited state 4FeO+ + CO sampled through reaction of ground state 4FeCO+ + N2O

The kinetics of the FeCO(+) + N2O reaction have been studied at thermal energies (300-600 K) using a variable temperature selected ion flow tube apparatus. Rate constants and product branching fractions are reported. The reaction is modestly inefficient, proceeding with a rate constant of 6.2 × 10(-11) cm(3) s(-1) at 300 K, with a small negative temperature dependence, declining to 4.4 × 10(-11) cm(3) s(-1) at 600 K. Both Fe(+) and FeO(+) products are observed, with a constant branching ratio of approximately 40:60 at all temperatures. Calculation of the stationary points along the reaction coordinate shows that only the ground state quartet surface is initially sampled resulting in N2 elimination; a submerged barrier along this portion of the surface dictates the magnitude and temperature dependence of the total rate constant. The product branching fractions are determined by the behavior of the remaining (4)OFeCO(+) fragment, and this behavior is compared to that found in the reaction of FeO(+) + CO, which initially forms (6)OFeCO(+). Thermodynamic and kinetic arguments are used to show that the spin-forbidden surface crossing in this region is efficient, proceeding with an average rate constant of greater than 10(12) s(-1).

Measurement of sodium-argon cluster ion recombination by coherent microwave scattering

This present work demonstrates a non-intrusive measurement of the rate constant for sodium-argon cluster ions (Na+·Ar) recombining with electrons. The measurements begin with resonance enhanced multi-photon ionization of the Na followed by coherent microwave scattering (radar) to monitor the plasma density. The Na+·Ar adduct was formed in a three-body reaction. The plasma decay due to recombination reactions was monitored as a function of time and modeled to determine the rate constant. At 473 K, the rate constant is 1.8−0.5+0.7×10−6cm3/s in an argon buffer at 100 Torr and initial Na number density of 5.5 × 1010 cm−3.

Reaction of the CF3 Radical with Various Cations (Ar+, Xe+, O2+, NO+, CO2+, C2F5+)

Abstract The kinetics of reactions of the trifluoromethyl radical (CF3) with several simple cations (Ar+, Xe+, O2+, NO+, CO2+, C2F5+) are studied via the variable electron and neutral density attachment mass spectrometry (VENDAMS) technique. CF3 is produced via dissociative electron attachment to CF3I, resulting in radical concentrations that are smaller than produced using pyrolysis methods, but better quantified. Rate constants and product branching fractions are reported, with typical uncertainties of ∼ 30%. Three of the reactions, Ar+, O2+, and C2F5+ proceed near the calculated collisional rate constant, while the other reactions are between 10% – 30% efficient. The inefficiency of the reactions of Xe+ and CO2+ can be explained by the reactions strictly obeying spin-conservation. The reaction of Ar+ yields predominantly (90%) CF2+ via dissociative charge transfer, with a minor channel to CF3+. The reaction of C2F5+ yields predominantly CF3+ with a second channel (∼ 25%) yielding C2F4+ and CF4, the only example here of a bond-breaking and bond-making reaction. For the other four reactions, only charge transfer to yield CF3+ is observed. Rate constants for these same ions with CF3I are also reported and found to be rapid with a variety of products formed.

Microwave scattering from laser spark in air

In this paper, microwave Mie scattering from a laser-induced plasma in atmospheric air is computed. It shows that the scattered microwave transitions from coherent Rayleigh scattering to Mie scattering based on the relative transparency of the laser-induced plasma at the microwave frequency. The microwave penetration in the plasma alters from total transparency to partial shielding due to the sharp increase of the electron number density within the avalanche ionization phase. The transition from Rayleigh scattering to Mie scattering is verified by both the temporal evolution of the scattered microwave and the homogeneity of polar scattering plots.

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.

Reducing the Breakdown Threshold in DC Microdischarges via Metal Nanoparticle Seeding

In this work, we present findings of significant reduction of the breakdown threshold in a DC microdischarge via seeding metal nanoparticles. Reductions in the breakdown voltage were found to be between 5 to 25% for PD values (the product of pressure and electrode gap distance) of 20 to 40 Torr-cm. These reductions were achieved be seeding aluminum and iron nanoparticles with mean diameters of 75 and 80 nm, respectively. This technique required no secondary energy source to achieve this breakdown threshold reduction. Through high-speed chemiluminescence imaging of the discharge evolution it was found that the increase in temperature following breakdown ignited some of the nanoparticles near the cathode. It is postulated that the breakdown threshold reduction is achieved by an effective reduction in the electrode gap distance due charging of the nanoparticles.

Sodium Cluster Ion Recombination Rate Measurements by Radar REMPI

This paper presents non-intrusive measurements of sodium-argon and sodium-nitrogen cluster ion recombination with electrons based on coherent microwave scattering (Radar) from Resonance Enhanced Multi-Photon Ionization (REMPI). The number density of sodium, in a mixture of sodium vapor with argon or nitrogen buffer gas, was determined by direct absorption measurement. Sodium was resonantly ionized by a tunable laser beam in the 2+1 REMPI process. Since the plasma is mainly made of sodium ions in the mixture, the sodium ion clusters are thus formed by the Chaperone process. The dissociative recombination rate of sodium-argon and sodium-nitrogen and the neutral stabilized recombination rate of sodium ions were determined from a least-squares Monte Carlo algorithm (LSM) that fitted the plasma dynamic model to the direct measurement of the total electron number in the laser-induced plasma. At 300 o C, and , the dissociative recombination rate of Na + ∙(Ar) was determined to be and the neutral stabilized recombination rate was determined to be . At 300C, and , the dissociative recombination rate of Na + ∙(N2) was determined to be and the neutral stabilized recombination rate was determined to be .The method can be easily extended to measurements of other cluster ion species over a wide range of temperatures and pressures in various mixture compositions, such as Mg and Ca with N2 and CO2. The results are of fundamental importance to the plasma chemistry field and hypersonic research.

Spatial and Temporal Evolutions of Microwave Scattering from Laser Spark in Air

In this paper phenomena seen in microwave scattering experiements from a laser-induced plasma in atmospheric air will be explained via Mie scattering theory. The relative transparency of the laser-induced plasma at microwave frequency is shown to lead to a transition from coherent Rayleigh scattering to Mie scattering. A sharp increase in conductivity during the avalanche ionization phase of the spark evolution alters the microwave penetration from total transparency to partial shielding. Temporal and polar scatterings plots will be presented to show the effects of the transition from Rayleigh scattering to Mie scattering and change in transparency.