Time-resolved, absolute HO₂ number density in diluted H₂-O₂-Ar, CH₄-O₂-Ar, and C₂H₄-O₂-Ar mixtures excited by a repetitive ns pulse discharge in a heated plasma flow reactor is measured by Cavity Ringdown Spectroscopy (CRDS). The experimental results are obtained at T=300-600 K and P=130 Torr, both during the discharge pulse burst and in the afterglow. The HO₂ number density is inferred from the CRDS data using a spectral model exhibiting good agreement with previous measurements of absolute HO₂ absorption cross sections. In the room-temperature H₂-O₂ mixture, as well as in CH₄-O₂ and C₂H₄-O₂ mixtures over the entire temperature range studied, HO₂ is generated only during the discharge burst and decays in the afterglow. However, in the H₂-O₂ mixture at elevated temperatures, T=400-600 K, HO₂ persists in the afterglow up to 10 ms after the discharge burst, comparable with the flow residence time in the reactor. Comparison with kinetic modeling shows that the sustained reactivity after the source of radicals is turned off is due to a chain propagation / hydrogen oxidation process, which dominates the radical recombination reactions. The kinetic modeling predictions are in good agreement with the relative HO₂ number density measured in all three mixtures, although the model unpredicts the absolute number densities in H₂-O₂ and CH₄-O₂ at T=400-600 K by up to a factor of two. Detection of the sustained low-temperature reactivity in H₂-O₂, initiated by the radical generation in the plasma, suggests that the plasma excitation may also affect kinetics of oxidation and reforming of fuels exhibiting low-temperature chemistry below hot ignition point. Development of a narrow linewidth optical parametric oscillator (OPO) for high-resolution, time-resolved HO₂ measurements during n-butane oxidation in a heated plasma flow reactor is underway.

Numerous spectroscopic techniques have been used to follow the concentration of reactive intermediates, such as free radicals, during chemical reactions. However, it is often necessary to measure absolute concentrations to test proposed kinetic mechanisms. Such measurements have always posed more of a challenge because of the difficulty of experimentally measuring the required value of the dipole transition moment to whose square the absorption cross section is proportional. In the-just presented talk involving HO₂, the issue was partially resolved since an empirical value of the absorption cross section was available¹ for the 2ν₂ overtone band. However, the cross-section is temperature and pressure dependent and the kinetic experiment was performed at different conditions than the cross section measurements. To resolve this problem, we used the pGopher software² with the HO₂ molecular constants³, μ, to simulate the HO₂ spectrum from which the cross sections were derived, and thereby get the transition moment, μ. A second simulation under the experimental conditions then determined the absolute HO₂ concentration. In addition, we carried out a quantum chemical calculation of μ using the CFOUR package. Despite the overtone nature of the transition, the calculation produced the same values of μ as our simulation, within experimental error. We are now investigating C₂H₅O₂ to compare experimental and calculated values of μ for its Ã−X̃ electronic transition. We hope to establish when it is reasonable to use calculated μ values instead of the difficult-to-measure, experimental ones, to determine concentrations of small, free radicals important in combustion and atmospheric reactions. ¹Thiebaud, et. al, J. Phys. Chem. A 2007, 111, 6959-6966; ² C. M. Western, J. Quant. Spectro. Rad. Tran., 186 221-242 (2017); ³DeSain, et. al, J. Mol. Spectro. 219 (2003) 163–16.

As the most abundant atmospheric hydrocarbon and potent greenhouse gas, methane plays a major role in the chemistry of Earth’s atmosphere. The study of kinetic isotope effects (KIEs) for H–D and ¹²C–¹³C substitutions on the reaction rates for methane is of importance for modelers, but is challenging because the effects are small, and producing methane oxidants results in a variety of secondary chemistry. In this work, single and double substitution methane KIEs for ¹³CH₄ and CH₂D₂ and were produced using the flash photolysis of N₂O and the methane isotopic compositions were determined via frequency-stabilized cavity ring-down spectroscopy using a dual wavelength near-IR DBF laser system. The KIE of ¹³CH₄ was found to differ with the literature value at room temperature of 1.013 by a factor of two, and the H-D KIE of CH₂D₂ was measured for the first time to be 1.098(23). Further, the expected lack of a temperature-dependence of the KIE over the range 170 – 300 K was seen as expected.

NO LIF/PLIF has been used extensively in visualizing complex structures of turbulent, reactive, and compressible flows. However, its quantitative measurement capability has been historically limited by the uncertainty in NO fluorescence quantum yield (FQY). In this work, we present a generic model for the FQY of NO A²Σ⁺(v’=0) based on state-to-state collisional energy transfer calculation. Two energy transfer pathways were considered in this model, namely the collisional quenching/de-excitation of the A²Σ⁺(v’=0) system down to X²Π(v”), and rotational energy transfer between various J levels (up to ∆J = 30) within A²Σ⁺(v’=0). Their respective rate constants were modeled using empirical expressions determined from previous data in the literature. Vibrational energy transfer, on the other hand, was not included in the current model, since its rate constants were several orders of magnitude lower than that of collisional quenching and rotational energy transfer. Both steady-state and time-dependent transient analyses, which correspond to pulsed-laser excitation and CW-excitation, respectively, were performed by solving the master equation of quantum state population. The results were compared with direct spectroscopic measurements of NO FQY conducted in a heated gas cell with various collision partners, which in turn, allowed iterative optimization of the quenching and rotational energy transfer rate model. The current model should be useful in predicting NO FQY over a wide range of temperatures and pressures.

The CPUF (Chirped Pulse in Uniform Flow) technique has been used previously in pulsed CRESU (Reaction Kinetics in Uniform Supersonic Flow) conditions to observe chemical reactions at low temperatures. We have adapted the technique to the continuous CRESU flows available in Rennes, and have observed the products of photolysis and chemical reactions using a chirped pulse Fourier transform millimeter wave spectrometer. We have characterized the flow conditions suitable for observing products of reactions and provide limits to the performance of these systems. In particular, pressure broadening is found to dominate these measurements, so steps had to be taken to resolve as much of the free induction decays as possible. We also observe the products of chemical reactions, particularly of CN radicals with hydrocarbons. The behavior of these products in CRESU environments as well as the results from these studies will be given and the application to observing the branching ratios of chemical reactions will be discussed.

In 1969 A. F. Andreev and I. M. Lifshitz radically changed the way we think about diffusion in cryocrystals by predicting that at sufficiently low temperatures the probability of exchange tunneling of neighboring particles in quantum crystals becomes noticeable such that impurities can move freely through the crystal as narrow-band quasiparticles.A. F. Andreev and I. M. Lifshitz, Sov. Phys. JETP. 29, 1107-1113 (1969).he term “quantum crystal” was introduced by de Boer in 1948 for substances in which the energy of the zero-point vibrations of the particles is comparable to the total energy of the crystal.J. de Boer, Physica 14, 139-148 (1948).he main idea put forth by Andreev and Liftshitz is that the rate of quantum diffusion should increase with falling temperatures and should show an inverse dependence on the concentration of impurities. As we will show, the hydrogen atom (H-atom) trapped in a parahydrogen crystal is an ideal candidate for quantum diffusion owing to its small mass and neutral charge. In 2013 our group published a communicationF. M. Mutunga, S. E. Follett, D. T. Anderson, J. Chem. Phys. 139, 151104-4 (2013).n the kinetics of the H + N₂O reaction in solid parahydrogen that showed an anomalous temperature dependence. In these studies we generate the H-atoms as byproducts of the in situ photodissociation of N₂O and monitor the subsequent reaction kinetics using rapid scan FTIR. Specifically, if we photolyze N₂O doped parahydrogen solids with 193 nm UV radiation at 4.3 K, we observe little to no reaction; however, if we then slowly reduce the temperature of the sample, we observe an abrupt onset to the reaction at temperatures below 2.4 K. In a number of studies conducted since this original work we have come to a better understanding of the effect of temperature on the reaction and will show data that the rate constant for the H + N₂O reaction shows an inverse dependence on the N₂O concentration. These findings support previous ESR measurements of H-atom quantum diffusion in solid parahydrogenT. Kumada et. al., J. Chem. Phys. 116, 1109-1119 (2002).nd more importantly illustrate how H-atom quantum diffusion impacts the kinetics of these anomalous low temperature, condensed phase reactions.

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Footnotes:

A. F. Andreev and I. M. Lifshitz, Sov. Phys. JETP. 29, 1107-1113 (1969).T J. de Boer, Physica 14, 139-148 (1948).T F. M. Mutunga, S. E. Follett, D. T. Anderson, J. Chem. Phys. 139, 151104-4 (2013).o T. Kumada et. al., J. Chem. Phys. 116, 1109-1119 (2002).a

Time-resolved, absolute number densities of ground state N atoms in nitrogen and H₂-N₂ plasmas, as well as ground state H atoms in H₂-N₂ plasmas, excited by a ns pulse discharge burst at P = 150 Torr, are measured by Two-Photon Absorption Laser-Induced Fluorescence (TALIF). Metastable N₂(A³Σ_u⁺,v=0,1) molecules and gas temperature in the discharge are measured at the same conditions by Tunable Diode Laser Absorption Spectroscopy (TDLAS). Both in pure nitrogen and in H₂-N₂ mixtures, the results show that the N atom number density does not decay on the time scale of 1 ms and is controlled only by the rate of N₂ dissociation in the plasma during the discharge burst. Both N₂(A³Σ_u⁺,v) populations and the rate of N atom generation decrease significantly during the ns pulse discharge burst (by a factor of 3-5), although the pulse energy coupled to the plasma and the number densities of N₂(C³Π_u) and N₂(B³Π_g) molecules remain approximately the same during the burst. Comparison of the measurement results and the modeling predictions indicates an additional major channel of N₂ dissociation in the plasma, by energy pooling in collisions of two N₂(A³Σ_u⁺) molecules: N₂(A³Σ_u⁺) + N₂(A³Σ_u⁺) → N + N + N₂. In H₂-N₂ mixtures, the N atom number density decreases significantly with the hydrogen mole fraction in the mixture, while the H atom number density is very high, up to 10¹⁶ cm⁻³ in a 1% H₂-N₂ mixture. Kinetic modeling suggests that a significant fraction of N atoms generated by electron impact in the plasma are formed in the excited electronic state, N(²D). In pure nitrogen, the electronically excited N(²D) atoms are quenched to the ground state, N(⁴S), within  ∼  10 μs, while in H₂-N₂ mixtures, they react with H₂ to produce NH and H atoms, N(²D) +H₂ → NH + H. Another major channel of H atom generation in the plasma is the reactive quenching of electronically excited nitrogen molecules by H₂, N₂(C³Π_u) + H₂ → N₂ + H + H. The rate of H atom generation remains nearly constant during the discharge pulse burst. The present work provides quantitative insight into the mechanism of radical species generation in N₂-H₂ plasmas, which is critical for understanding the kinetics of ammonia synthesis by non-thermal plasma-assisted catalysis.

Strong vibrational nonequilibrium is sustained in nitrogen and nitrogen/carbon dioxide “hybrid” plasmas, generated by a ns pulse train overlapping with a sub-breakdown RF waveform. N₂ vibrational level populations in the plasma are measured by broadband Coherent Anti-Stokes Raman Spectroscopy (CARS). Vibrationally excited CO₂ and CO in the plasma are detected by Quantum Cascade Laser Absorption Spectroscopy (QCLAS). The plasma is generated using a custom-designed external circuit which overlaps a ns pulse train and a sine-wave RF waveform, generated by two different power supplies, in plane-to-plane, double dielectric barrier discharge configuration. The results show that the sub-breakdown RF waveform, which does not produce ionization but heats the electrons generated by the ns pulses, has a strong effect on vibrational excitation of molecules in the plasma. This enables isolating and quantifying the contribution of reactions of vibrationally excited molecules in plasmachemical and plasma-catalytic processes. Specifically, the effect of the excitation of CO₂ asymmetric stretch vibrational mode on the dissociation of carbon dioxide is studied by QCLAS measurements of CO₂ vibrational level populations and CO product. In a closely related study, a quasi-two-dimensional atmospheric pressure plasma jet in a noble gas/nitrogen mixture, impinging on a dielectric plate (glass or quartz), is interrogated by Coherent Anti-Stokes Raman Spectroscopy (CARS). The experiments are made in N₂/Ar, N₂/He, and N₂/Ne plasma jets powered by a ns pulse train or an RF waveform. Strong N₂ vibrational nonequilibrium is measured in the jet, and found to increase as the mole fraction of nitrogen in the mixture is reduced. This suggests that N₂ vibrational excitation is produced by the energy transfer from metastable electronically excited atoms to the nitrogen vibrational mode, possibly via the metastable excited electronic state, N₂(A³Σ_u⁺). To quantify the role of electronically excited nitrogen in this energy transfer processes, the number density of N₂(A³Σ_u⁺) molecules in the plasma is measured by Tunable Diode Laser Absorption Spectroscopy.

Absolute time-resolved populations of nitrogen molecules in a metastable excited electronic state, N₂(A³Σ_u⁺), generated in a repetitive ns pulse discharge in reacting gas mixtures, have been measured by Tunable Diode Laser Spectroscopy (TDLAS). N₂(A³Σ_u⁺,v=0-1) population measurements are made in CO₂-N₂, CH₄-N₂, CH₄-CO₂-N₂, and C₂H₄-N₂ mixtures at a pressure of 150 Torr. The mixtures are excited in a diffuse plasma generated by a repetitively pulsed, double dielectric barrier, ns discharge across a 6 mm gap in a plane-to-plane geometry. The data are taken during the discharge bursts up to several tens of pulses long. The results show that the N₂(A³Σ_u⁺) number density generated in the discharge, both by electron impact and by the cascade quenching of the higher energy excited electronic states, are significantly affected by the mixture components. In CO₂-N₂ mixtures, the effect on N₂(A³Σ_u⁺,v) populations is relatively minor, due to the relatively slow quenching rate. In CH₄-N₂ mixtures, the dominant effect is due to the vibrational energy transfer to methane, resulting in a rapid vibrational relaxation of N₂(A³Σ_u⁺,v=1 → v=0). In CH₄-CO₂-N₂ mixtures, N₂(A³Σ_u⁺) quenching is also affected by the species generated in the plasma. The results are compared with kinetic modeling predictions, identifying the mechanisms of N₂(A³Σ_u⁺) generation and decay during the discharge pulses and in the afterglow. The results demonstrate that high-pressure, high repetition rate, volume-scalable ns pulse discharges can be used for efficient generation of atomic and radical species for plasma chemical and plasma catalysis syntheses. N₂(A³Σ_u⁺) measurements and kinetic modeling analysis can be used to quantify the amount of reactive species generated in the plasma.

Thermal decomposition of cyclohexane at temperatures up to 1310 K was performed using flash pyrolysis coupled with vacuum ultraviolet (118.2 nm) photoionization time-of-flight mass spectrometry. The experimental results revealed that the major initiation reaction of cyclohexane decomposition was C-C bond fission leading to the formation of 1,6-hexyl diradical. The 1,6-hexyl diradical could isomerize to 1-hexene and decompose into •C₃H₇ + •C₃H₅ and •C₄H₇ + •C₂H₅. The 1,6-hexyl diradical could also undergo direct dissociation; the C₄H₈ fragment via the 1,4-butyl diradical intermediate was observed, serving as evidence of the 1,6-hexyl diradical mechanism. Quantum chemistry calculations at UCCSD(T)/cc-pVDZ level of theory on the initial reaction pathways of cyclohexane were performed and found to be consistent with the experimental conclusions. Cyclohexyl radical was not observed as an initial intermediate in the pyrolysis. Benzene was produced from sequential H₂ eliminations of cyclohexane at high temperatures.

We are utilizing chirped pulse mm-wave spectroscopy in quasi-uniform supersonic flows at low temperature to probe isomer-specific product branching and kinetics. Earlier we have studied the photodissociation of the propargyl radical and monitored the branching between the three isomers of C₃H₂ and noted efficient conversion of H₂CCC and HCCCH to the cyclic form induced by H addition/elimination, a process termed H-catalyzed isomerization. We will show that by adding a D atom source to the flow we observe very efficient D atom enrichment in the products through an analogous process of D addition/H elimination to C₃H₂ isomers occurring at 40K or below. Cyclic C₃HD is the only deuterated isomer observed consistent with the expected addition/elimination yielding the lowest energy product. In the high-density region of the flow, we also observe singly deuterated propyne formed following stabilization of the D + C₃H₃ adduct, and both isotopomers CH₂DCCH and CH₃CCD are observed in nearly equal abundance. The implications of this for deuterium fractionation in astrochemical environments will be further discussed with the support of monodeuterated dark cloud model developed by Millar et al. Recent results for low temperature radical-radical reactions relevant to planetary atmospheres will also be highlighted.

Reaction dynamics of the radical-radical reaction between the resonantly-stabilized propargyl radical with NH₂ and ND₂ were investigated using chirped-pulse rotational spectroscopy in a pulsed quasi-uniform flow. Propargyl, NH₂, and ND₂ radicals were produced by UV photodissociation of a mixture of propargyl bromide and ammonia in a low-temperature, helium Laval flow. The reaction products, HCN, HNC, HC₃N, DCN, DNC, and DC₃N, were simultaneously identified using their vibrational ground state rotational spectra in the 70 – 92 GHz region and product branching ratios were calculated. A theoretical analysis using RRKM calculations on an ab initio potential surface revealed diverse and complex reaction pathways and mechanisms that lead to the observed reaction products.

Vibrationally excited oxygen and nitrogen have been long recognized to be of critical importance in nonequilibrium high-enthalpy flows, primarily encountered behind hypersonic shock waves. To date, no direct experimental verification of vibrational state resolved dissociation rates predicted by different kinetic models has been undertaken, in large part due to the difficulty of measuring these rates directly. We present a framework where state specific recombination rates can be inferred from the time-resolved measurements of O₂ vibrational populations, such that the state specific dissociation rates can be obtained from the detailed balance. In the present approach, recombination reactions of atomic oxygen are monitored in the afterglow of a diffuse ns pulse discharge burst in an O₂-Ar mixture. The time evolution of O₂(v) populations in the recombining mixture are measured by ps Laser Induced Fluorescence (LIF) in the O₂(B³Σ_u⁻,v’=0 ← X³Σ_g⁻, v”) Schumann-Runge bands, with absolute calibration by NO(A²Σ⁺, v’=0 ← X²Π, v”=0) LIF. By varying the output wavelength of the ps laser / Optical Parametric Oscillator (OPO) system used for the excitation in the 250-450 nm range, levels from v”=7 to 21 have been detected. Two-photon Absorption LIF (TALIF) at 226 nm has been used to measure the atomic oxygen concentration, with absolute calibration by Xe. Within  ∼ 1 ms after the discharge burst, a rapid decay of O₂(v) is observed, indicating vibration-vibration (V-V) and vibration-translation (V-T) relaxation of vibrational states populated by electron impact and by quenching of the excited electronic states of Ar. After the rapid initial decay, the vibrational populations level off and remain nearly constant, or exhibit a transient rise, on the timescale of about 10 ms, suggesting the presence of a persistent source of vibrational excitation due to chemical reactions. In addition to O atom recombination to form vibrationally excited O₂ molecules, at low temperatures atomic oxygen may also recombine with O₂ and form ozone. The latter may well generate vibrationally excited O₂ in O + O₃ → O₂ + O₂ reaction, which would therefore affect the present results. Ozone formation is controlled by preheating the excited mixture up to 600 K, as well as increasing the pressure and reducing the O₂ mole fraction, when the production of ozone is reduced significantly.