Low-energy collision phenomena in free jet expansions: Molecular relaxation theory and ion-molecule rate studies.

Abstract

Theoretical and experimental development of a new kinetic method to measure the rate coefficients of ion-molecule reactions occurring in free jet expansions below 20K is presented. The method is successfully used to determine the temperature dependences of numerous bimolecular and termolecular ion-molecule reactions over the temperature range of 0.5-20K. A new theoretical method based on the generalized Boltzmann equation is developed to calculate macroscopic flow properties of pure molecular supersonic flows. The variation of the different temperature components, hydrodynamic speed and density of the free jet as a function of distance is presented assuming a Maxwellian anisotropic distribution function. This theory facilitates the kinetic analysis and the assignment of temperatures to the chemical reactions occurring in jets. Using the Boltzmann equation, the flow properties of a mixed atomic free jet expansion are also analyzed. The method is more general than previous treatments which assume a vanishingly small mole fraction for one component of the mixture. The presence of velocity slip arising from the difference in hydrodynamic speeds of the two components complicates this treatment. Expressions for the calculation of flow properties for an atomic mixture with an arbitrary composition are presented. Temperature dependences of the termolecular association rate coefficients for the reactions of, N₂⁺ + 2N₂, O₂⁺ + 2O₂ and NO⁺ + 2NO over the temperature range of 3-15K are presented. The results are discussed in the light of statistical phase space theory. For the reactions of N₂⁺ + 2N₂ and O₂⁺ + 2O₂ excellent agreement between theory and experiment is obtained. The kinetic analysis of NO⁺ + 2NO is complicated due to the competing charge transfer reaction. The observed temperature dependence for this reaction does not agree with the predictions of the statistical theory. The ternary association rate coefficients for the reaction, Ar⁺ + 2Ar, show a strong temperature dependence at very low temperatures (0.5-2.5K). Current statistical formulations cannot predict this temperature dependence and a comprehensive model for this reaction mechanism has yet to be developed. Three distinct temperature dependences are observed for the bimolecular reactions of N₂⁺ with CH₄, O₂ and n-H₂ at temperatures below 15K. Speculations are made regarding the interaction potential energy surfaces that may lead to the observed behaviors.