J. V. Michael

Pressure Dependence and Third Body Effects on the Rate Constants for H+O2, H+NO, and H+CO

This paper presents measurements of the third order rate constants for the reactions H+O2+M→ HO2+M, H+NO+M→ HNO+M, H+CO+M→ HCO+M. The measurements were made by concentration analysis of hydrogen atoms by means of Lyman α photometry. All experiments were performed at room temperature. Particular emphasis was placed on the effect of changing the third body and, therefore, experiments were performed with M being H2, He, Ne, Ar, and Kr for all three reactions.

Reaction H+C2H4: Comparison of Three Experimental Techniques

The reaction H+C2H4 has been studied as a function of He pressure at room temperature with three independent experimental techniques, and rate constants have been obtained in both the excess ethylene and excess hydrogen‐atom environments. The products of the reaction are CH3, CH4, and C2H6. An experimental value for the third‐body recombination coefficient for H+CH3+M has been obtained. The over‐all stoichiometry of the reaction has been experimentally determined and is found to vary as a function of the initial reactant concentrations. A reaction mechanism is proposed which accounts for all of the experimental data obtained.

Reaction H+C2H4: Investigation into the effects of pressure, stoichiometry, and the nature of the third body species

The detection of H atom concentrations by Lyman α photometry has been employed in a time resolved experiment to obtain the pressure dependence of the apparent bimolecular rate constant for the reaction of H atoms with C2H4 at room temperature. A computer simulation analysis has been applied to adjust the observed rate constants for H atom depletion in reactions subsequent to the initial reaction. The pressure falloff behavior at low pressures of various heat bath species He, H2, N2, Ar, Ne, Kr, and SF6 were also obtained. Experiments at high pressures of He have permitted an extrapolation to the high pressure limit of the rate constant of (16.1± 3.2) × 10−13 cc molecule−1· sec−1.

Absolute Cross Section for Hg(3P1) + H2 and the Imprisonment Lifetimes for Hg(3P1) at Low Opacity

Steady‐state hydrogen‐atom concentrations produced by the mercury‐photosensitized decomposition of molecular hydrogen are detected by Lyman‐α photometry. On the basis of the natural lifetime of Hg(3P1) and the mechanism for the reaction, quenching plots are constructed, and the over‐all bimolecular rate constant for Hg(3P1) + H2 is calculated. The absolute cross section for Hg(3P1) + H2 is determined to be 10.8 ± 0.4 A2 in the limit of low mercury opacity. This result agrees with other recent experimental determinations. In addition, experimental values of imprisonment lifetimes for Hg(3P1) as a function of mercury concentration are estimated and are compared with other low‐opacity and high‐opacity results. The results are also compared to various theoretical predictions of imprisonment lifetimes.

Experimental Estimate of the Oscillator Strength of the P2 32,12 ←S2 12 Transition of the Hydrogen Atom

The oscillator strength of the Lyman-α transition in the hydrogen atom has been determined by the line-absorption method. Hydrogen atoms were produced in a fast-flow system by a microwave discharge in a H2–He mixture. The hydrogen atoms absorbed the Lyman-α radiation at 1216 A emitted by a hydrogen-neon lamp, and absolute atom concentration was determined by titration with NO2. On the basis of the photometric calibration curve determined in this way, oscillator strengths were calculated as a function of assumed emission-line profiles. The best fit to the experimental data yielded an oscillator strength of 1.09±0.27.

An investigation of nonequilibrium kinetic isotope effects in chemically activated ethyl radicals

The reactions of thermal H and D atoms with C2H4 and C2D4 have been studied in helium diluent at room temperature with a discharge flow system time of flight mass spectrometric apparatus. New absolute measurements of the apparent bimolecular rate constants, under atom excess conditions, for the reactions D+C2H4, H+C2D4, and D+C2D4 have been obtained as a function of pressure. Mechanistic studies have also been carried out for the D+C2H4 and H+C2D4 reactions in the pressure region of 1–5 torr. The observed pressure dependence of the rate constants and the mechanisms for the two mixed cases have been found to differ markedly. The variation in over‐all mechanism for the mixed isotope cases, ethyl‐d1 and ethyl‐d4, and the nonequilibrium kinetic isotope effects for chemically activated ethyl radicals are presented and compared with other work. RRKM theoretical calculations are shown to be consistent with the experimental observations for the four systems.

Application of RRKM theory to the chemical and thermal activation of ethyl radicals

The quantum statistical RRKM theory of unimolecular reactions has been applied to the decomposition of excited ethyl radicals. These radicals can be formed either by collisional activation in a thermal system or chemically by the addition of hydrogen atoms to ethylene molecules. The assessment of pertinent parameters has been based on an appraisal of theory and experiment. The pressure dependence of the rate constant for the chemical activation reaction was used to assist in the assignment of activated complex parameters. These parameters were varied until good agreement was obtained between the calculated pressure falloff and data from this laboratory for the H+C2H4 reaction. The final parametric assignments were then used in calculations at higher temperatures for comparison with the thermal decomposition data for ethyl radicals from other laboratories.