Temperature dependences for the reactions of O2−and O−with N and O atoms in a selected-ion flow tube instrument

Kinetics of associative detachment of O- + N2 and dissociative attachment of e- + N2O up to 1300 K: chemistry relevant to modeling of transient luminous events

The rate constants of O- + N2 → N2O + e- from 800 K to 1200 K and the reverse process e- + N2O → O- + N2 from 700 K to 1300 K are measured using a flowing afterglow - Langmuir probe apparatus. The rate constants for O- + N2 are well described by 3 × 10-12 e-0.28 eV kT-1 cm3 s-1. The rate constants for e- + N2O are somewhat larger than previously reported and are well described by 7 × 10-7 e-0.48 eV kT-1 cm3 s-1. The resulting equilibrium constants differ from those calculated using the fundamental thermodynamics by factors of 2-3, likely due to significantly non-thermal product distributions in one or both reactions. The potential surfaces of N2O and N2O- are calculated at the CCSD(T) level. The minimum energy crossing point is identified 0.53 eV above the N2O minimum, similar to the activation energy for the electron attachment to N2O. A barrier between N2O- and O- + N2 is also identified with a transition state at a similar energy of 0.52 eV. The activation energy of O- + N2 is similar to one vibrational quantum of N2. The calculated potential surface supports the notion that vibrational excitation will enhance reaction above the same energy in translation, and vibrational-state specific rate constants are derived from the data. The O- + N2 rate constants are much smaller than literature values measured in a drift tube apparatus, supporting the contention that those values were overestimated due to the presence of vibrationally excited N2. The result impacts the modeling of transient luminous events in the mesosphere.

Chemical kinetics in an atmospheric pressure helium plasma containing humidity

Atmospheric pressure plasmas are sources of biologically active oxygen and nitrogen species, which makes them potentially suitable for the use as biomedical devices. Here, experiments and simulations are combined to investigate the formation of the key reactive oxygen species, atomic oxygen (O) and hydroxyl radicals (OH), in a radio-frequency driven atmospheric pressure plasma jet operated in humidified helium. Vacuum ultra-violet high-resolution Fourier-transform absorption spectroscopy and ultra-violet broad-band absorption spectroscopy are used to measure absolute densities of O and OH. These densities increase with increasing H2O content in the feed gas, and approach saturation values at higher admixtures on the order of 3 × 1014 cm-3 for OH and 3 × 1013 cm-3 for O. Experimental results are used to benchmark densities obtained from zero-dimensional plasma chemical kinetics simulations, which reveal the dominant formation pathways. At low humidity content, O is formed from OH+ by proton transfer to H2O, which also initiates the formation of large cluster ions. At higher humidity content, O is created by reactions between OH radicals, and lost by recombination with OH. OH is produced mainly from H2O+ by proton transfer to H2O and by electron impact dissociation of H2O. It is lost by reactions with other OH molecules to form either H2O + O or H2O2. Formation pathways change as a function of humidity content and position in the plasma channel. The understanding of the chemical kinetics of O and OH gained in this work will help in the development of plasma tailoring strategies to optimise their densities in applications.

Kinetics of CO+ and CO2+ with N and O atoms

We have measured reaction rate constants for CO⁺ and CO₂⁺ reacting with N and O atoms using a selected ion flow tube apparatus equipped with a microwave discharge atom source. Experimental work was supplemented by molecular structure calculations. Calculated pathways show the sensitivity of kinetic barriers to theoretical methods and imply that high-level ab initio methods are required for accurate energetics. We report room-temperature rate constants of 1.0 ± 0.4 × 10^-11 cm³ s^-1 and 4.0 ± 1.6 × 10^-11 cm³ s^-1 for the reactions of CO⁺ with N and O atoms, respectively, and 8.0 ± 3.0 × 10^-12 cm³ s^-1 and 2.0 ± 0.8 × 10^-11 cm³ s^-1 for the reactions of CO₂⁺ with N and O atoms, respectively. The reaction of CO₂⁺ + O is observed to yield O₂⁺ exclusively. These values help resolve discrepancies in the literature and are important for modeling of the Martian atmosphere.