Reactions and Reaction Kinetics at Elevated Pressures

One of the early investigations at pressures above 0.1 Torr was a study of ethylene by Field. By operating at pressures up to about 0.35 Torr, a number of ions were observed that could only be formed by reactions of order three or greater. Field was unable to determine appearance potentials at such elevated pressures with sufficient reliability to employ this method for the identification of precursors and in most instances it was not possible to distinguish the reaction or reactions leading to a certain ion. However, by plotting ratios of product ions as a function of pressure, he was able to show that products of reactions up to sixth order were observable. 

With higher-order reactions, there is a question of the mechanism of formation of ions of higher order. Thus, 

Obviously, a ternary ion, T, might be formed by reaction of PM or of with M. By considering reaction orders, stoichiometry, and thermochemical data. Field concluded that with all of the reactions for which he had sufficient data to reach a conclusion, the ternary ion was formed by col- 

If it can be assumed that the intermediate, although long-lived, still decomposed rapidly compared to the rate of collision, one can safely assume that the steady-state criterion can be employed in the calculation of rate constants. Under these conditions, it is easily shown that the rate of a ternary reaction to form would be as follows  

From Eq. (29), it is obvious that a plot of (Ti )/(Aq) against (M) will yield the overall third-order rate constant. By employing this method. Field determined the rate constant for several third-order reactions and in the systems under investigation these all fell in the range 1-8 x 10 cc molecule sec When it is recalled that the rate constant for the three-body recombination of most atoms is approximately 10 , it is evident that these are comparatively rapid third-order processes. This is not surprising since the complex in each case contains many more vibrational modes than is the case for the recombination of two atoms. First-order rate theory shows that the rate of the decomposition of a molecule decreases as the number of vibrational modes increases. Thus, Field s observations are at least qualitatively what would be expected. Subsequent investigators have found similar rate constants for ternary reactions. 

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