Temperature and internal state dependence of ultralow energy ion-neutral reactions.

Abstract

This dissertation presents results on the study of the temperature and internal state dependence of ion-neutral reactions. The free jet flow technique is used to measure rate coefficients for several reactions at ultralow collision energies near 1 K. The technique, and the unique considerations of free jet flow are considered. The method of analysis of the data obtained from the free jet reactor is also presented. The measurement of reaction rate coefficients for several fast reactions is reported. These studies demonstrate the utility of the technique as various types of reactions which occur at the collision rate are studied. Reactions which do not occur at the collision rate have also been studied. Several slow reaction rate coefficients of the atomic ion AR⁺ are measured, and the data acquired from the free jet flow reactor aids in the elucidation of the reaction mechanisms for these systems. The slow reaction between C₂H₂⁺ and H₂ is also considered, and a theory to account for its unusual temperature dependence is presented which depends heavily on the formation of a long lived collision complex. The experimental rate coefficients for three body association reactions of the rare gas atomic ions Ar⁺, Kr⁺ and Xe⁺ are presented. The experimental results in this case show very large rate coefficients which cannot be explained satisfactorily by any current theories. Using resonantly enhanced multiphoton ionization to create quantum state specific ions, the measurement of rate coefficients for selected vibrational states of molecular ions and spin orbit states of atomic ions are reported. Observed effects for vibrational excitation of molecular ions and spin-orbit excitation of atomic ions are discussed. Finally, the production and subsequent dynamics of negative ions by electron attachment are examined. The electrons are produced from a high resolution source by using two color resonantly enhanced multiphoton ionization spectroscopy on a suitable precursor.