Low Temperature Collisions and Reactions in a 22-Pole Ion Trap

This work consists of two major parts, a theoretical and an experimental one. The latter part investigates ion molecule reactions performed in a temperature variable 22-pole ion trap. The former one analyses different aspects of radio frequency ion traps and the dynamics of charged particles located inside. Therefore, fundamental concepts for the description of charged particles in oscil- lating electrical fields are discussed on as a first step (Chapter 3). The calculation of these fields via different numerical methods is elaborated on subsequently (Chap- ter 4). The boundary element method (BEM) has proven highly adequate for the calculation of electrostatic potentials. Hence it is depicted in detail. Using analytically solvable electrostatic problems, the previously calculated fields will be tested with respect to their accuracy. To perform the necessary calculations both own numeric routines have been developed and available routines have been employed. This work shows how the numerically, calculated fields can be converted into an analytical description via appropriate multipole expansion. This permits an efficient simulation and investigation of the dynamics of charged particles in ion traps. Simulations of different existing multipole ion trap geometries help to analyze the behavior of ions in oscillating fields with respect to, inter alia, the increase of their translational energy, their energy distribution and shifts of the secular frequency caused by higher anharmonicities (Chapter 5). Furthermore, space charge effects and perturbations of the fields due to mechanical misalignments of electrodes are included here. Consequently, this work is meant to contribute to the optimization of existing ion traps and to the development of new and better ones. The experimental part focuses on the reaction of nitrogen ions with hydrogen molecules taking into account the fine structure state of N + and the nuclear-spin state of H 2 . This reaction can be considered as the first process or step in the formation of interstellar ammonia. Up to date this very fundamental reaction – although having been studied in several laboratories for decades – is not fully understood. For example, it remains unclear whether or not the reaction really endothermic or it is just inhibited by a barrier. Also, it is not known to what extent the fine structure state energy N + favours/promotes this reaction. All these questions are of great relevance for low temperature astrochemistry. Applying a non-adiabatic reaction model and a global fit, fine structure state specific rate coefficients for the reaction mentioned above could be derived (Chapter 7). Initially, a large number of test measurements were conducted in order to clarify the impact of different measurement parameters like e.g. temperature or number density on measured rate coefficients (Chapter 6).On the basis of these test measurements probable conclusions of initial difficulties with reproducing test results could be drawn. Thereupon, experimental processes were optimized to minimize errors in determination the rate coefficients. As a result,+ rate coefficients for the reaction of N + 2 und CO with hydrogen in a temperature range between 10 to 120 K were determined as well as ternary rate coefficients for the formation of nitrogen-helium-clusters (NHe n ) + (n = 1, 2) in a temperature range between 10 – 13 K (Chapter 9). Also, measurements were performed with respect to fine structure state changing collisions of the nitrogen ion due to helium. Moreover, this work contains the characterization of a piezoelectric valve which was performed using the reaction of Ar + ions with H 2 (Chapter 10). The purpose of the characterization is to develop a new experimental method for the study of ion molecule reactions. This aims at separating translational thermalization of ions as well as their relaxation of the internal excited states from the reaction process. A quantitative model could be developed in order to describe the reaction gas flow as a function of time, temperature and reaction gas pressure