G. Kowarik

Hot electrons induced by slow multiply charged ions

The dissipation of energy following the impact of multiply charged ions on a polycrystalline metal surface was studied using thin film metal–insulator–metal junctions as targets. The ions hit the top Ag layer of a Ag–AlOx–Al junction, where they excite electrons and holes. A substantial fraction of these charge carriers is transported across the insulating barrier and can be detected as an internal current in the bottom Al layer. The effects of potential and kinetic energies on this tunneling yield are investigated separately by varying the charge state of the Ar projectile ions from 2+ to 8+ for kinetic energies in the range from 1 to 12 keV. Per impinging ion yields of typically 0.1–1 electrons are measured within the thin film tunnel junction. The tunneling yield is found to scale linearly with the potential energy of the projectile. In addition, the tunneling yield shows a strong dependence on the internal barrier height which can be modified by an external bias voltage.

Low-energy ion-induced electron emission in metal-insulator-metal sandwich structures

An Ag-AlO{sub x}-Al sandwich structure is used to investigate the electronic excitation induced by Ar{sup +} ions at the surface of the top 15 nm Ag film. The internal electron emission yield, i.e., the number of electrons emitted per impinging ion into the bottom Al film, is determined as a function of the kinetic energy of the ions in the range of 300-6000 eV. A comparison to the external electron emission yield, i.e., the number of electrons per projectile ejected into the vacuum, reveals two interesting aspects. First, unlike in the external emission, no significant contribution of the potential energy to the internal electron emission yield is observed. Second, the kinetic part of the electron emission yield exceeds the external one over the entire energy range. Another interesting result is that the internal emission yield shows a power-law dependence on ion kinetic energy. A Monte Carlo simulation, based on a simple theoretical treatment of the kinetically induced electron emission, supports the experimental findings. Finally, we discuss the influence of excitation properties (e.g., anisotropy) as well as of device properties (e.g., film thickness, barrier height) on the computed electron emission yields.

Nanostructures induced by highly charged ions on CaF2 and KBr

Impact of a highly charged ion upon a solid surface can induce dramatic changes in the morphology only by the release of its potential energy. Hillocks and mono-atomic deep pits have been observed on the surfaces of CaF2 and KBr, respectively. For both processes a threshold in the potential energy exists for the creation of these nanostructures. Above this threshold the structure size increases linearly with potential energy. The mechanisms for the formation of hillocks and pits are discussed and a first attempt to present a unified microscopic picture is made.

Guiding Of Slow Highly Charged Ions Through A Single Mesoscopic Glass Capillary

We present experimental studies of the transmission of slow 4.5 keV Ar9+ ions through a single cylindrical‐shaped glass capillary of macroscopic dimension with large aspect ratio. We find stable transmission of a micrometer‐scale beam with considerable intensity after a charge‐up phase, in which a self‐organized process leads to the formation of a guiding electric field. We show the time evolution of the transmitted intensity through the sample for various capillary tilt angles.

Potential energy - induced nanostructuring of insulator surfaces by impact of slow, very highly charged ions

We have recently shown that the impact of individual slow highly charged ions is able to induce permanent nano-sized hillocks on the surface of a CaF2 single crystal. The experimentally observed threshold of the projectile ion potential energy necessary for hillock formation could be linked to a solid-liquid phase transition (nano-melting). In this contribution we report on similar nano-sized surface modifications as a result of the potential energy of impacting highly charged ions for other surfaces.

Transmission of 4.5 keV Ar9+ ions through a single glass macrocapillary

Recently, we were able to show the validity of the so-called guiding effect of highly charged ions, known from insulating nanocapillaries, up to a macroscopic length-scale of straight capillaries. The self-organized formation of charge-patches leads to a guiding electric field inside the glass capillary, causing stable transmission after a charge-up period. Our early investigations were mainly focused onto the quasi-static transmission behaviour. In this contribution we are going to present further systematic data on the quasi-stable transmission regime as well as the time-evolution of the transmission.

The effect of temperature on guiding of slow highly charged ions through a mesoscopic glass capillary

We present first temperature dependent transmission measurements of slow highly charged ions through a single, straight Duran glass capillary with a high aspect ratio. By changing the temperature of the glass capillary the electrical conductivity of the Duran can be varied by several orders of magnitude. This held the promise to investigate the effect of conductivity on particle transport (build-up and removal of charge patches) through capillaries in more details.

Single and double electron capture by slow He2+ from atoms and molecules

Impact of slow alpha particles (He2+) on H2 and O2 is relevant for radiation cooling in future magnetically confined burning fusion plasmas. A new compact experimental setup has been utilized for measuring absolute cross sections for single (SEC) and double electron capture (DEC), and transfer ionization (TI) in collisions of slow (impact energy typically 0.1–1 keV times ion charge) singly and multiply charged ions with atoms and molecules. Our method combines collection of slow product ions and electrons with primary ion beam attenuation and stopping in a differentially pumped target gas chamber. Reliability of the new setup has been checked by measuring well established SEC and DEC cross sections for impact of slow singly and doubly charged noble gas ions on their atoms (He, Ne, Ar). We have then measured absolute cross sections for SEC by both 3He2+ and 4He2+ from Ne, and in further consequence will study fusion relevant SEC and DEC by He2+ from H2 and O2.