Interaction of slow multicharged ions with solid surfaces

This topical review focuses on recent advances in the understanding of the formation of surface nanostructures, an intriguing phenomenon in ion–surface interaction due to the impact of individual ions. In many solid targets, swift heavy ions produce narrow cylindrical tracks accompanied by the formation of a surface nanostructure. More recently, a similar nanometric surface effect has been revealed for the impact of individual, very slow but highly charged ions. While swift ions transfer their large kinetic energy to the target via ionization and electronic excitation processes (electronic stopping), slow highly charged ions produce surface structures due to potential energy deposited at the top surface layers. Despite the differences in primary excitation, the similarity between the nanostructures is striking and strongly points to a common mechanism related to the energy transfer from the electronic to the lattice system of the target. A comparison of surface structures induced by swift heavy ions and slow highly charged ions provides a valuable insight to better understand the formation mechanisms.

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Single ion induced surface nanostructures: a comparison between slow highly charged and swift heavy ions

We report the formation of highly oriented, uniform, and spherical nanoparticles of 3C–SiC as a result of Coulomb explosion during the interaction of near-infrared ultrafast laser pulses with 3C–SiC thin films grown on Si substrate. Experiments were performed at laser fluences well below the single shot, thermal modification threshold.

Coulomb explosion-induced formation of highly oriented nanoparticles on thin films of 3C–SiC by the femtosecond pulsed laser

The potential energy stored in multiply charged ions is liberated when the ions recombine during impact on a solid surface. For certain target species this can lead to a novel form of ion–induced sputtering, which, in analogy to the usual kinetic sputtering, has been termed ‘potential sputtering’. This sputtering process is characterized by a strong dependence of the observed sputtering yields on the charge state of the impinging ion and can take place at ion–impact energies well below the kinetic sputtering threshold. We summarize a series of recent careful experiments in which potential sputtering has been investigated for hyperthermal highly charged ion's impact on various surfaces (e.g. Au, LiF, NaCl, SiO2, Al2O3 and MgO), present the different models proposed to explain the potential sputtering phenomenon and also discuss possible applications of potential sputtering for nanostructure fabrication.

Potential sputtering: One contribution of 11 to a Theme 'Sputtering: past, present and future. W. R. Grove 150th Anniversary Issue'.

The mention of few‐electron atoms usually brings to mind hydrogen, helium or other light elements in neutral form. However, these simple atoms are part of a sequence of ions having the same number of electrons but different nuclear charges. For example, the hydrogen‐like sequence spans neutral hydrogen through hydrogen‐like uranium, U91+. Both the atomic physics and the applications of the most highly charged ions in such isoelectronic sequences are receiving increasing attention. Recently the electronbeam ion trap has made it possible to produce and study any such ion in a modest‐sized apparatus (figure 1).

The Electron‐Beam Ion Trap

First results from a series of experiments aimed at the study of the interaction of slow very highly charged ions with conductor and insulator surfaces are reported. Charge state dependences of secondary electron, x-ray and secondary ion emission are measured. In addition, microscopic studies are performed using an ‘Atomic Force Microscope’ to investigate surface defects, produced through the stored electrostatic potential of the incident ion. The ions that have been used for these studies range from O7+ to Th80+, with low kinetic energies (generally 1–3 keV/amu). Overall, enhancement of the low energy secondary electron, sputter ion and photon emission are observed with increasing charge states. Saturation in the electron emission yield at very low velocities confirm present models for image charge acceleration effects. X-ray emission spectra are found to be consistent with the formation of so called ‘hollow’ atoms near the surface during the neutralization processes. The microscopic studies rev...

Ion/surface interaction studies with 1-3 keV/amu ions up to Th80+

The angle and energy dependence of electron emission following the interaction of 70--170-MeV ${\mathrm{Ne}}^{\mathit{q}+}$ (q=7,10) projectiles with 20 and 100 \ensuremath{\mu}g/${\mathrm{cm}}^{2}$ carbon foils has been investigated experimentally. Absolute emission yields were determined for ejection angles ranging from 0\ifmmode^\circ\else\textdegree\fi{} to 180\ifmmode^\circ\else\textdegree\fi{} and electron energies from 10 eV to 6 keV for normal-incidence ion beams. The experimental \ensuremath{\delta}-electron spectra are compared to results from a newly developed theoretical model based on a separation of energy loss and angular scattering for secondary electrons.

Investigation of delta -electron emission in collisions of highly charged fast Ne projectiles with carbon-foil targets.

Particle emission induced by the interaction of highly charged slow Xe-ions with a SiO2 surface

Absolute doubly differential electron emission yields were measured as a function of projectile scattering angle for 30 to 300 keV H++He and Ar++Ar collisions. Coincidence spectra were taken for projectile scattering angles between 0.3° and 3°, electron energies in the range of 5 eV to 1000 eV, and ejection angles from 40° and 110°. In the case of 300 keV H++He collisions, triple coincidences were performed in order to measure electron spectra as a function of the impact parameter and final target charge‐state. The importance of higher‐order effects in the description of the well‐known 4fσ‐promotion in 150‐300 keV Ar++Ar collisions is investigated. Furthermore, comparisons with experiment show the limits of single‐center atomic orbital expansions and of the independent‐particle model in the case of H++He collisions.

D. Schneider

Papers

Ion/surface interaction studies with 1-3 keV/amu ions up to Th80+

Investigation of delta -electron emission in collisions of highly charged fast Ne projectiles with carbon-foil targets.

Particle emission induced by the interaction of highly charged slow Xe-ions with a SiO2 surface

Multiply differential ionization probabilities in small impact parameter ion‐atom collisions