Lifetime of a freely decaying hollow atom

Hollow atoms (HAs) are an exotic type of matter formed when a highly charged ion impacts on a surface, capturing multiple electrons into highly excited states, leaving intermediate electronic states empty. Although experimental fingerprints of HAs were found in high-resolution x-ray spectra, it has been widely believed that HAs decay too rapidly to be studied directly. Using a simulation code for the full deexcitation cascade of the HAs, based on rates from atomic structure codes for highly excited states, we show that the lifetime of an HA, scattered under very grazing angles from a surface, can reach several tens of ps in free decay. In additional experiments, we use <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msup><a:mrow><a:mi>Ar</a:mi></a:mrow><a:mrow><a:mn>14</a:mn><a:mo>+</a:mo></a:mrow></a:msup></a:math> ions on a Ni(110) surface under incidence angles below <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mn>0</b:mn><b:mo>.</b:mo><b:msup><b:mn>5</b:mn><b:mo>∘</b:mo></b:msup></b:mrow></b:math>. Comparing ion charge state distributions after scattering between experiment and simulation, we discuss the role of surface-stimulated decay in most interactions. We show that by minimizing the incidence angle, the influence of surface-near processes can be reduced and that HAs can exist for several picoseconds in free space.