Interaction of Slow Highly Charged Ions with Surfaces

Abstract

Interaction of a slow highly-charged ions (HCIs) with metal and semiconductor surfaces is discussed with particular attention on findings uniquely obtained with microcapillary targets and hydrogen-terminated Si surfaces. The microcapillary targets enable for the first time extraction of hollow atoms (ions), i.e., atoms (ions) in highly-and multiply-excited states, into vacuum. The intrinsic nature of hollow atoms (ions) was studied through X-ray and visible light measurements. X-ray measurements revealed that a considerable fraction of charge-changed ions was in meta-stable states keeping innershell holes with lifetimes of ~ns or longer, i.e., metastable multiply excited states were effectively formed. Spin-aligned states with less than half-filled innershells and/or highly excited high angular momentum states were proposed as possible candidates to explain such extreme metastability. The angular distributions of these metastable ions were as narrow as those of the incident HCIs, i.e., a high quality beam of hollow atoms (ions) can be prepared with the microcapillary technique. Visible light measurements provide detailed information on electronic states formed at the beginning of the HCI-surface interactions. It was found that principal quantum numbers of the states were around the incident charge states, which is consistent with the prediction of the classical over barrier model. Proton sputtering from well-defined H-terminated Si surfaces were studied. It was found that the proton yields were proportional to q ~ 5. Desorption induced with double electron transfer from a chemical bond was proposed as a principal mechanism of the proton sputtering with HCIs.