Hollow atom spectroscopy

Abstract

A slow multicharged ion (MCI) approaching a solid surface will be converted by multiple electron capture from the latter into a transiently multiply excited, dynamically neutral atom. The resulting "hollow atom" decays rapidly in autoionisation cascades which give rise to abundant emission of slow electrons. Upon its arrival at the surface, the hollow atom will be rapidly squeezed to a much smaller size because of shielding by the quasi-free target electrons, then further de-excited and eventually fully relaxed below the surface. During these later de-excitation phases further slow electrons are emitted in conjunction with fast Auger electrons and soft x-ray photons, which result from recombination of projectile inner shell vacancies. All these processes, which take place during the rather short lifetime (typically picoseconds) of the hollow atom, can be elucidated by spectroscopy of the emitted electrons and soft x-ray photons, and from the kinematics (scattering angles) and dynamics (charge states and energy losses) of projectiles scattered on atomically flat target surfaces. Based on presently available experimental evidence, generation and decay of these hollow atoms can be reasonably well described within a semi-classical "over-the-barrier" model.