D. D. Davis

Flash photolysis‐resonance fluorescence kinetics study: Temperature dependence of the reactions OH + CO → CO2 + H and OH + CH4 → H2O + CH3

Two reactions involving the transient chemical species OH(ν″ = 0) and the reactants CO and CH4 have been investigated over a temperature range of nearly 140°C. Of particular importance were the measurements made below 300°C where data has heretofore been lacking. The rate constant expressions in Arrhenius form are kCO = (2.15±0.19)×10−13 exp[−(160±80 cal mol−1)/R T] and kCH4 = (2.36±0.21)×10−12 exp[−(3400±175 cal mol−1)/R T]. Units are cm3 mol−1 · sec−1. Wide variations in the total pressure, H2O concentration, initial OH concentration, and the nature of the diluent gas showed no indication of complex secondary reactions. The activation energies reported in this work for both processes, virtually zero for Reaction (1) and 3400 cal mol−1 for Reaction (2), are incompatible with those same quantities previously reported at elevated temperatures and indicate either nonlinear Arrhenius behavior for these OH reactions or possibly errors in experimental measurements.

Absolute rate constants for the reaction O(3P)+NO2→NO+O2 over the temperature range 230–339°K

Using the technique of flash photolysis‐resonance fluorescence, absolute rate constants have been measured for the reaction O(3P)+NO2→ NO+O2. Over the temperature range 230–339°K, the rate constant was found to have the value k=9.12± 0.44 × 10−12cm3 molecule−1sec−1, independent of temperature. At stratospheric temperatures, this rate constant is about a factor of two faster than indicated from previous measurements.

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.

A kinetics study of the reaction of OH radicals with two C2 hydrocarbons: C2H4 and C2H2

The flash photolysis‐resonance fluorescence technique has been utilized to study the kinetics of hydroxyl radical reactions with ethylene and acetylene at 300 K over a wide range of experimental conditions. (1) OH+C2H4 →k1 Products (e.g., C2H5O), (2) OH+C2H2 →k1 Products (e.g.,C2H2O+H). The bimolecular rate constant for Reaction (1) was observed to increase from (2.24–5.33) × 10−12 cm3 molecule−1⋅ sec−1 as the total pressure varied from (3–300) torr of helium. The rate of Reaction (2) was invarient with total pressure, and the value obtained for k2 was (1.65±0.15) × 10−13 cm2 molecule⋅sec−1. The observed pressure dependency of Reaction (1) brings the existing literature values for k1 into close agreement. Reaction (2) has been shown to be particularly sensitive to secondary processes and this is discussed in some detail.

Direct rate measurements showing negative temperature dependence for reaction of atomic oxygen with cis‐2‐butene and tetramethylethylene

Reported in this paper are the first direct rate measurements showing a negative temperature dependence for the reaction of ground state atomic oxygen with cis‐2‐butene and tetramethylethylene. Wide variations made in the experimental conditions (e.g., total pressure, O atom concentration, and olefin concentration) of these two systems have shown that the measured rate constants were uninfluenced by secondary reactions. The absence of any dependence of the measured rate constants on total pressure at several temperatures indicate that the reactions investigated were bimolecular processes. When expressed in the form of an Arrhenius equation, the observed negative temperature dependence results in an apparent negative activation energy, i.e., kcis‐2‐B=(9.69± 0.96)× 10−12exp (319± 63 cal mol−1/RT) and kTME=(5.58± 1.07)× 10−12 exp (1570± 120 cal mol−1/RT). Units are in cubic centimeters per molecule seconds. If a threshold energy of 0.0 cal mole−1 is assigned to the reaction of O(3P) with TME, the temperature...

Electronic quenching and vibrational relaxation of the OH (A 2Σ+, v′=1) state

Rate constants for electronic quenching and vibrational relaxation of the v′=1 manifold of (A 2Σ+) OH by Ar, H2, N2, and He have been obtained by a laser fluorescence method. The quenching rate constants for v′=1 are not appreciably different from those which have been measured for v′=0. The relaxation rate constants are a factor of 3 larger than the quenching rate constants for Ar and H2, and 5 times larger for nitrogen. (AIP)