Ion formation in doped helium nano-droplets

This thesis presents a series of experiments that study sequential ionization processes of embedded atoms and molecules by helium atoms in a long-lived excited state. Helium nano-droplets provide a favourable environment for the formation of more than one metastable atom. In this work I report on the formation of doubly charged species caused by collision of excited atoms with ions. Doping superfluid helium nano-droplets with methyl iodide and exposing them to electrons enhances the formation of doubly charged iodine atoms at the threshold for the production of two metastable helium atoms. These observations point toward a novel ionization process where doubly charged ions are produced by sequential Penning ionization. The process is both fundamental and universal and should also occur in different environments with different excited atoms or excited molecules. In addition, studies of small helium cluster cations are presented in this doctoral work. The high mass resolution of our apparatus makes it possible to fully resolve small helium cluster ions from impurities and to unambiguously identify abundance anomalies in the cluster size distribution. Magic numbers at n=7,10 and 14 were identified. Furthermore, the enhancement in the yield of the helium cation tetramer relative to other small cluster ions was affirmed in our measurements when the stagnation conditions in the cluster source are changed from sub-critical to supercritical expansion. (author)