Interaction of Slow HCI with Solid Surfaces: What Do We Know, What Should We Know?

Abstract

A slow multicharged ion (MCI) which approaches a solid surface catches electrons from the latter into highly excited states and transiently becomes a neutral “hollow atom” (HA). Rapid autoionisation of this HA will be balanced by further ongoing electron capture until close surface contact where shielding sets in, and inner-shell vacancy recombination at and below the surface produces further slow electrons and characteristic fast electrons and soft X-rays. Until its complete neutralization the projectile continues to be attracted toward the surface by its decreasing image charge which causes a minimum impact velocity. Many aspect of this HA scenario have been studied by a number of groups for different ion species and - charge states at metal-, semiconductor- and insulator surfaces with a number of experimental methods. The most detailed comparison with available theory is possible if electron emission and projectile energy losses and -charge changes are studied in mutual coincidence for scattering along well defined trajectories at smooth mono-crystalline target surfaces. For insulator surfaces so called “potential sputtering” (PS) and strong secondary ion emission can be induced by the multi-electron capturing MCI. Our current understanding of the above-surface part of MCI-surface interactions is satisfactory and can also explain how “free” hollow atoms and -ions are produced by funneling MCI through thin capillaries or colliding them on clusters or fullerenes. Because of the approximately additive influence of potential- and kinetic ion energies on the resulting electron emission, for this part a straighforward definition of the term “highly charged ion” (HCI) can be given. On the other hand, processes occuring at and below the surface are so far less well understaood since they depend on the potential- and kinetic ion energies in a clearly non-additive way. This is particularly true for PS and excitation of target plasmons due to the potential MCI energy. PS and related phenomena are also of practical interest for new surface-analytical and nano-technological applications.