Cold molecular ion-neutral collisions in a dynamic ion-atom hybrid trap

This thesis explores the fundamental physical and chemical processes between molecular ions and neutral atoms in the gas phase at very low energies. At temperatures in the mK regime, these are dominated by long-range interactions and only a few partial waves. This study contributes to the understanding of the details of intermolecular interactions and provides valuable data for benchmarking theoretical models and quantum-chemical calculations. Although there is a wide range of previous atomic collision system studies, the investigations of cold collisions with molecular ions are limited. The experimental setup used in this work enabled the measurement of rate coefficients for molecular ion-neutral collisions. The development of a dynamic ion-neutral hybrid trap with improved control over collision energy led to new insights into charge-transfer rate coefficients for molecular ion-neutral atom collisions involving N$^+_2$+Rb, O$^+_2$+Rb and N$_2$H$^+$+Rb. Two different experimental settings enable state-and collision-energy dependent measurements of rate coefficients for all systems revealing an intriguing interplay between long-and short-range effects for the homonuclear diatomics by comparison to theoretical calculations and surprising experimental results for the polyatomic study. A fit of a constant function to the measured CT rate coefficients in the state-dependent experiment of N$_2$H$^+$+Rb yielded the similar result as for N$^+_2$+Rb($5s$)($^2S_{1/2}$). In the collision-energy dependent experiment the CT rate coefficient for N$_2$H$^+$+Rb($5s$)($^2S_{1/2}$) and N$_2$H$^+$+Rb($5p$)($^2P_{3/2}$) are both similar to the results for N$^+_2$+Rb($5s$)($^2S_{1/2}$), and also the rate coefficients increase as the collision-energy increases.