Dynamics of highly charged ions interacting with surfaces

PhD ceremony: Mr. E. Bodewits, 14.45 uur, Academiegebouw, Broerstraat 5, Groningen

Thesis: Dynamics of highly charged ions interacting with surfaces

Promotor(s): prof. R.A. Hoekstra

Faculty: Mathematics and Natural Sciences

When slow, highly charged ions interact with surfaces, they interact in general only with the topmost layer of the surface. During this interaction many different and complex processes occur. Although a lot of research has been done on the interaction of HCI’s interacting with surfaces, there remain many open questions, for example the exact role of the surface electronic structure. In order to get a better understanding of the processes which take place when an ion approaches a surface, several experiments have been done and described in this thesis. Upon approaching the surface, a highly charged ion is neutralized within a few to tens of femto–seconds only. The ions will resonantly capture electrons from the surface at a certain distance above the surface, which, depending on the charge state of the ion, can be up to many tens of atomic units. These electrons are not captured into the inner shells but in the outer shells, thereby creating the so called hollow atoms. Relaxation of this hollow atom then takes place by means of Auger processes. Even though this scenario is nowadays widely accepted, the details of the electron dynamics are not yet fully known. In order to get a better understanding of these dynamics, the electrons and (x–rays) emitted during relaxation of the hollow atom can be used as a diagnostics tool.

In this thesis two different topics have been addressed which provide more insight into these processes. The first topic addresses the dynamics of the guiding capacities of nano-capillaries are studied. The nano-capillary targets exist out of circular holes through an insulating foil, with typical capillary diameters of hundreds of nanometers and lengths in the order of tens of micrometers i.e. a factor of 100 longer than wide. These nano-capillaries exhibit the remarkable property of deflecting the trajectories of the incoming ions towards their main axis. The deflection is caused by charge patches inside the nano-capillary which are created by the ion beam itself. Therefore, the deflection occurs without the need for any external electric or magnetic fields.

The second topic addressed in this thesis is the secondary electron emission from highly charged ions interacting with surfaces. The electrons are emitted by a variety of processes. These processes can be divided into two major classes: kinetic emission and potential emission. The kinetic emission occurs when a particle hits the surface with enough energy and transfers this energy to the surface in order to emit electrons. The potential emission however is driven by the potential energy an ion carries. In order to be able to measure electron yields, from as well kinetic as potential emission, a new setup called IISIS was designed, constructed and tested at the KVI. First, as an experiment to test the new setup, Ar and Xe ions with different charge states ranging from q=4+ to q=26+ impinging on a clean Au(100) surface were used. The measured electron yields turn out to depend linearly on the potential energy of the incoming HCIs, in line with earlier experiments. Also on an absolute scale the determined secondary electron yields agree with existing data. This leads to the conclusion that the experimental setup performs as expected and can be used reliably for measurements of electron yields.