Walter Braun

Intense vacuum ultraviolet atomic line sources.

Intense atomic lines (O, N, S, C, Br, Cl, H, Se, Kr) have been produced by microwave excitation of mixtures of various gases in helium under flow conditions. The intensities generally obtained are greater than 10(14) quanta/sec and are suitable for atomic emission studies and as photochemical light sources in the vacuum uv. Conditions for producing these high purity line sources are discussed.

Flash Photolysis of Methane in the Vacuum Ultraviolet. II. Absolute Rate Constants for Reactions of CH with Methane, Hydrogen, and Nitrogen

By means of kinetic spectroscopy, the concentration of CH has been measured in the flash photolysis of methane. Measurements were made by following the attenuation of C 2Σ+←X 2Π Q branch at 3143 A, where the disappearance of CH is of the first order in CH. Experiments were conducted with pure methane, methane+H2, and methane+N2. The reactions and the corresponding rate constants in mole−1 cm3 sec−1 are CH+CH4→C2H4+H,k=1.5×1012;CH2+H2→CH3,k=6.2×1011;CH+N2→products,k≃4.3×1010;CH+CH→C2H2,k≃1.2×1014.

Survey of Photochemical and Rate Data for Twenty‐eight Reactions of Interest in Atmospheric Chemistry

Photochemical and rate data have been evaluated for 28 gas phase reactions of interest for the chemistry of the stratosphere. The results are presented on data sheets, one per reaction. For each reaction, the available data are summarized. Where possible there is given a preferred value for the rate constant or, for the photochemical reactions, preferred values for primary quantum yields and optical absorption coefficients.

Flash photolysis of carbon suboxide: absolute rate constants for reactions of C(3P) and C(1D) with H2, N2, CO, NO, O2 and CH4

The vacuum ultraviolet flash photolysis of C3O2 in the 159.0 nm absorption band has been investigated. The major primary products are C(1S), C(1D), C(3P), and CO. The species C2 and C3 have also been observed but are of minor importance in the overall reaction scheme. A number of pressure independent reactions involving C(3P), C(1D), and C(1S) with CO, CH4, N2, NO, O2, and H2 have been observed by means of the kinetic-spectroscopic method. The rate constants measured at room temperature are summarized here (cm3 s-1 molecule-1): C(3P) + CH4 → C2H4 (?) k < 5 x 10-15 (7) C(1D) + CH4 → C2H2 + H2 k = 3.2 x 10-11 (8) C(1D) + N2 → C(3P) + N2 k ≈ 2.5 x 10-12 (10) C(3P) + NO → CN + O k = 1.1 x 10-10 (12) C(1D) + NO → CN + O k = 9.2 x 10-11(13) C(1D) + H2 → CH + H k = 4.15 x 10-11 (18) C(1S) + H2 → CH + H(?) k < 5 x 10-12 (19) C(3P ) + O2 → CO + O k = 3.3 x 10-11 (20) C(1D) + O2 → CO + O(?) k < 5 x 10-12 (22) The pressure dependent reaction rates of C(3P) with N2, CO, and H2 have been qualitatively measured and are discussed in detail.

Absolute Rate Constants for the Reaction of Atomic Oxygen with Ethylene over the Temperature Range 232–500°K

Rate constants for the reaction of atomic oxygen with ethylene were measured over a temperature range of 232–500°K using the flash photolysis‐resonance fluorescence technique. The rate constant at room temperature was also determined using a flash photolysis‐kinetic absorption spectroscopy system and a discharge‐flow system coupled to a mass spectrometer. Within the experimental errors of the three techniques, good agreement was found for the rate constant at 298°K. The bimolecular rate constant was also found invariant to changes in both total pressure and reactant concentration. Over the temperature range of the experiments, the rate data could be fitted by a simple Arrhenius expression of the form, k=5.42± 0.30× 10−12 exp[(−1130± 32 cal mole−1)/RT]cm3molecule−1· sec−1.

Absolute Rates of the Reactions H+C2H4 and H+C2H5

The hydrogen atom–ethylene system was studied at 298°K employing the methods of resonance fluorescence and absorption by hydrogen atoms of Lyman α radiation at 1216 A. The contribution of hydrogen atom–radical reactions was evaluated under varying experimental conditions, and the rate of disappearance of H atoms in ethylene was measured under conditions where stoichiometric corrections became significant. Measurements in the literature of reaction rates for H+C2H4 at low total pressure are now in good agreement; however the limiting high‐pressure absolute rate constants thus far reported differ depending on the assignment of stoichiometric factors. Our results indicate that stoichiometric factors obtained under low‐pressure conditions may not be applicable to high pressure. Furthermore, extrapolations based on plots of inverse rate constant vs inverse pressure may be in error due to significant curvature in such plots. Our high‐pressure limiting rate constant for H+C2H4, extrapolated from data at pressure...

Absolute Rate of the Reaction H+H2S

Flash photolysis coupled with resonance fluorescence of Lyman‐α radiation at 121.6 nm has been used to investigate the rate of reaction of H atoms with H2S over the temperature range 190–464°K. Conditions were chosen under which atom–radical and radical–radical reactions were unimportant and only the H‐atom–H2S reaction occurred. The rate constant thus obtained can be expressed as k1 = (1.29 ± 0.15) × 10−11exp[− (1709 ± 60) / 1.987T] cm3molecule−1·sec−1. Comparison of the Arrhenius A factor with that predicted by entropy considerations suggests a somewhat loose activated complex, but not as loose as expected on the basis of the exothermicity of the H+H2S reaction.

Infra-red laser enhanced reactions: chemistry of vibrationally excited O3 with NO and O2(1Δ)

Abstract Vibrationally excited ozone, produced by absorption of CO2 laser radiation, was found to react significantly faster with NO and O2(1Δ) than thermal ozone. Using a modulation technique, absolute and relative rate constants at 300K for the following reactions were calculated assuming rapid equilibration between the three closely spaced vibrationally excited levels of O3, and that only the lowest level of these, the ν2 bending mode, is active in reaction. k1′ + k2′ = 2.7 × 10−13 cm3 molecule−1 s−1; (k1′ + k2′)/(k1 + k2) = 16.2 ± 4.0; k1′/k1 = 4.1 ± 2.0; k2′/k2 = 17.1 ± 4.3; k7′/k7 = 38 ± 20. These rate constants must be modified if a different combination of vibrationally excited levels is involved. The fraction of vibrational energy usable in chemical reaction was found to be about 15, 50 and ∼ 100% respectively for processes 1′, 2′ and 7′. Our measurements clearly differentiate between the participation of vibrational energy and thermal energy but do not distinguish differences between the individual vibrationally excited states. Details of the modulation technique, involving chemiluminescence detection of NO2 and resonance fluorescence detection of oxygen atoms, are described. Comparison of our results with a previous measurement of the summation reaction (1′ + 2′) shows excellent agreement.

Infrared laser enhanced reactions: Temperature resolution of the chemical dynamics of the O3† + NO reaction systen

The rate constant for the decay of vibrationally excited ozone, O3†, in the O3† + NO reaction system has been measured from 153 to 373 K. Vibrationally excited O3 was produced in the asymmetric stretch normal mode by absorption of square wave modulated emission from a CO2 laser tuned to the P (30) 9.6 μm transition. Under appropriate experimental conditions, a rapid V→V coupling process involving all three normal modes of O3 is believed to set up a Boltzmann population distribution among them. Reaction or relaxation of O3+ out of this subset of normal modes is observed to proceed through a weighted average of rate constants. From the effects of temperature and buffer gas pressures an assessment can be made as to the predominant loss mechanism for the various modes. While there are three separate convolution schemes which appear to fit our data, we are persuaded to emphasize one whereby all three modes contribute via a reaction channel described by kD = (2.0×10−11) exp(−1525/T) cm3 molecule−1⋅sec−1 while ν...

Flash Photolysis of Methane in the Vacuum Ultraviolet. I. End‐Product Analysis

Methane and mixtures of methane and methane‐d4 have been subjected to flash photolysis in the vacuum ultraviolet. Ethylene is the major hydrocarbon product. Isotopic distributions of the hydrogen, ethane, and ethylene fractions as well as complete product analysis suggest that CH plays a dominant role in the photolysis. The CH is formed either by direct dissociation of excited methane or secondary flash photolysis of CH2 in the flash. On the basis that acetylene is formed with unit efficiency by association of CH and ethylene is formed by reaction of CH with methane, the collision yield of the latter reaction is about 1/80.