Low Pre-exponential Factor Research Articles

Polyfluoroalkyl derivatives of nitrogen. Part XXXIII. The kinetics of the reaction between trifluoronitrosomethane and tetrafluorethylene to form perfluoro-2-methyl-1,2-oxazetidine

The kinetics of the reaction between trifluoronitrosomethane and tetrafluoroethylene to give perfluoro-2-methyl-1,2-oxazetidine have been studied for both the gaseous and liquid phase reactions, with mixture compositions in the range 1 : 5 to 5 : 1 trifluoronitrosomethane:tetrafluoroethylene. The gas-phase reaction has been studied at temperatures between 110 and 150° with total initial pressures in the range 125–500 Torr and, with 1,1,2-trichloro-trifluoroethane as solvent, the liquid-phase reaction has been examined between –35 and 80° over a four-fold variation in the initial total concentration of reactants. The reaction is a homogeneous bimolecular reaction in both phases, and the respective second-order rate constants are given by the Arrhenius equations: k(l mol–1 s–1)=(3·85 ± 0·78)× 107 exp –(16,200 ± 250/RT), k(l mol–1 s–1)=(3·27 ± 0·54)× 106 exp –(13,215 ± 90/RT). It is suggested that the abnormally low pre-exponential factors obtained reflect the formation of the transition state in a specifically orientated collision process, and that the nature of this transition state is similar to those obtained in Diels–Alder addition reactions.

Investigation of point defects in silicon and germanium by non-irradiation techniques

Abstract The paper reviews the information on vacancies and interstitials from such techniques as diffusion studies, precipitation from supersaturated solid solutions, quenching from high temperatures, and plastic deformation. Self- and impurity diffusion are considered in some detail and the evidence is presented according to which the high-temperature self-diffusion occurs via extended interstitials in silicon and via extended vacancies in germanium. From the diffusion coefficients of group-V impurities and from the precipitation of substitutional nickel impurities the selfdiffusion coefficient for a vacancy self-diffusion mechanism is derived and found to be lower than that from high-temperature self-diffusion experiments. This constitutes evidence for a change-over in the self-diffusion mechanism in silicon at about 900 °C to a low-temperature vacancy mechanism with lower activation energy and lower preexponential factor than the interstitial mechanism. It is proposed that self-interstitials in silicon might have a rather high migration energy, E 1I,M; tentatively; E 1I M ⋍ 0.8 eV and a donor level at 0.40 eV above the valence band are attributed to the self-interstitials in silicon.

ESR Study of the Kinetics of the Reaction of H Atoms with Methane

A wide-temperature-range study using ESR atom detection has been employed to measure the rate of the reaction H + CH4→H2 + CH3. Over the temperature range 426°–747°K, we obtain a specific rate constant for the above reaction of k1 = 6.9 ± 0.6 × 1013exp[(−11 800 ± 200) / RT] expressed in units of cubic centimeters per mole per second. The value obtained in the present work is compared to a number of other results obtained by various workers. The activation energy we observe is considerably higher than previous values obtained in this temperature range. However, our results coupled with the heat of the reaction predict an activation energy for the reverse reaction which agrees well with experimental values. In addition, the pre-exponential factor we obtain agrees with absolute rate theory predictions as well as with entropy considerations. We do not agree with literature results which give very low pre-exponential factors suggesting steric factors of the order of 10−3–10−5 for this reaction.

Radiolysis of liquid mercury di-n-hexyl

The radiolysis of liquid mercury di-n-hexyl gives mercury, hexane, hexene and dodecane as the principal products, indicating that only mercury—carbon bond cleavage takes place. The ratio of disproportionation to combination of n-hexyl radicals in this environment is 0.73. A study of the effect of varying the dose-rate on the system indicates that the chain reaction C6H13˙+Hg(C6H13˙)2→C6H14+C6H12+Hg+C6H13˙ takes place. The rate constant for this reaction is estimated as 38.1. mole–1 sec–1 and is low because of a low pre-exponential factor.

On field evaporation

Abstract The theory of field evaporation outlined in previous publications (Muller 1956, Gomer and Swanson 1963, Brandon 1965 a) has been extended to include the effects of image gas bombardment of the specimen and has been experimentally tested for the metals W, Mo and Pt in the range 60 to 90°K. In general the experimental results are consistent with the theory, polarization corrections amounting to some 15% of the total energy required to evaporate the doubly charged metal ions. The most serious discrepancies were observed in the case of molybdenum and are associated with plastic deformation occurring during the field evaporation process and with the presence of tightly bound non-metallic impurities. Deviations from the simple theory, in particular low pre-exponential factors in the rate equations, are associated with partial tunnelling of the metal ions through the activation energy barrier.

Recombination of Vacancies and Interstitials in KBr at Low Temperatures

Measurements and a detailed analysis were made of the thermal annealing of the $\ensuremath{\alpha}$ band in KBr x rayed at temperatures below 20\ifmmode^\circ\else\textdegree\fi{}K. Four distinct annealing stages at 11, 17, 19, and 21\ifmmode^\circ\else\textdegree\fi{}K were resolved, the first three stages corresponding to first order reactions and the fourth stage to a second order reaction. The activation energies for the annealing stages were about 0.015, 0.03, 0.04, and 0.06 eV, respectively. The first three stages were interpreted as a correlated recombination of close pairs of bromide vacancies and interstitials, and the fourth stage as the recombination of more distant defect pairs through a free migration of an interstitialcy. The activation energy 0.06 eV is interpreted as the free migration energy of a bromide-ion interstitialcy. Interaction energies between a vacancy and an interstitial ion in several configurations were calculated and possible annealing sequences for the first three annealing stages are proposed. The observed low pre-exponential factor for the first stage is also discussed.