Stabilized Hollow Ions Extracted in Vacuum

$K$ x rays emitted from $2.1\mathrm{keV}/\mathrm{u}\phantom{\rule{0ex}{0ex}}{\mathrm{N}}^{6+}$ ions passed through a thin Ni microcapillary foil were measured in coincidence with the exit charge states. Ions with a $K$ hole but with several electrons in outershells, i.e., hollow ions formed above a surface (in the first generation), were successfully extracted in vacuum. It was found that a considerable fraction of extracted hollow ions had extremely long lifetimes of the order of ns.