The study of multiply-charged ion-solid interactions is of considerable technological importance for the understanding of material damage, surface modification, and plasma-wall interactions. The recent availability of sources for slow highly charged ions (HCI), namely electron cyclotron resonance (ECR) and electron beam ion sources (EBIS) has led to a flurry of research activities, both experimental and theoretical, in the field of HCI-solid interactions. On the most fundamental level, its importance is derived from the complex many-body response of surface electrons to the strong Coulomb perturbation.

From numerous experimental as well as theoretical studies the following scenario of the HCI-surface interaction has emerged: When an HCI approaches a solid surface, one or more electrons are resonantly captured at large distances into high Rydberg states of the projectile. As a result, so-called hollow atoms (ions) are formed where the atomic charge cloud transiently resides in shells with large diameters while the core is virtually empty. Direct observation of this short-lived state is complicated by the fact that the ion is always attracted towards the surface by its self-image potential. Consequently it will suffer close collisions upon impact on the surface and the memory of the hollow atom is all but erased. This problem has motivated the study of interactions of HCI with internal surfaces of microcapillaries and nanocapillaries as an alternative technique to study above surface processes.

Metal and insulating capillaries have become available at the Tokyo Metropolitan University, Japan, and at the Hahn-Meitner-Institut Berlin, Germany. The use of capillary targets allows the extraction of hollow atoms in vacuum. Observation of photons or Auger electrons emitted from them in flight becomes possible. Also the energy loss an HCI suffers when passing through a capillary at distances too large for charge transfer to take place can be measured and calculated.