Energy spectrum of field ionization at a single atomic site

Energy deficit spectra of field ions coming from above a single atomic site are measured by using an atom-probe FIM modified with a Möllenstedt energy analyzer. This device offers a resolution of 5 × 10^?5 and is inherently more efficient and less noisy than a retarder. The energy spectra made up of 10 to 100 ions/sec are displayed on the screen of an assembly of two microchannel plates and are photographically recorded within a few seconds. Jason peaks for H₂⁺ and Ne⁺ are confirmed, and are also found for He⁺. High-order multiple peaks appear when ions are taken from the flat, closely packed net planes of W and Ir field ion emitters. The results are in quantitative agreement with a resonance model similar to one by Alferieff and Duke and by Jason. Noble gas ions are also observed from the forbidden zone near the surface, and interpreted as apex-adsorbed atoms ionized by or after excitation by the electron shower coming from farther-out field ionization of other gas atoms. Energy from excited metastable apex-adsorbed atoms may account for artifact vacancies observed particularly when field evaporation is performed in neon.