Ph. D. Thesis: Transmission of slow highly charged ions through nano-structures

by Mr nadeem akram (stockholm university)

Thursday 24 May 2012, 13:00 → 17:00 Europe/Stockholm

FA31

This thesis is based on experimental investigations of transmitting slow highly charged ions through nano-structures of various cross-sections. Transmission through rhombic and rectangular nanocapillaries in muscovite and phlogopite mica, respectively, is used to study the guiding and shaping of highly charged ion beams. The two-dimensional angular distributions of the transmitted ions reveal that slow highly charged ion beams are tailored into rectangular and rhombic shapes after passing through the capillaries of rhombic and rectangular cross-sections, respectively. These transmission profiles are maintained for tilt angles within the geometrical opening angle of the capillaries. The ‘incident charge-dependent’ time evolution of the transmission profiles indicates that the tailored shape comes from the image force experienced by the traversing ions and the deposited charge by the incident ions moves the transmission profiles towards higher observation angles with respect to the beam direction. Transmission is also observed for tilt angles larger than the geometrical opening of the capillaries and evidence of charging-up of capillary walls was seen. Other incident charge-dependent features including the increase in angular width and shift of transmitted angular distributions were also observed. Starting from initially charged capillaries, it was found that the deposited charge must be distributed uniformly along the capillary walls to maintain the tailored rhombic shape through rectangular capillaries. In this thesis, a technique is presented which is successfully employed to investigate directly the formation of charge patches on the walls of a glass capillary by imaging the visible photons emitted due to impact of ions on the walls. These tapered glass capillaries were applied in biological studies of cell damage by ion impact and the technique provides a new way to directly observe the development of ion-guiding in these capillaries. With the help of this technique the mechanism of ion-guiding can be controlled and optimized. We also review the transmission characteristics of slow highly charged ions through nanometer thick foils and present the results of transmission of slow highly charged ions through ultra-thin carbon nano-sheets of molecular layer thickness. The observed energy loss is smaller than the calculated one using SRIM and agrees rather well with the Firsov model. The transmitted ions also keep their initial charge state up to 98% in a complete contradiction to the electron capture rate predicted by the classical over-the-barrier model. The results suggest that the energy loss of slow highly charged ions in such thin sheets is due to the electronic excitations, without charge exchange inside the target.