How can a neutral atom capture an electron to continuum states?

Abstract

We report on a systematic experimental study of the cusp peak associated with the ionization of the target (known as electron capture to the continuum, ECC) by impact of neutral atoms. In a measurement carried out for He 0 on Ar collisions, the dependence of ECC on the impact energy was investigated. The energy was varied between 18.9 and 250 keV/amu. The relative contributions (“fractions”) of the different processes (electron capture and loss to the continuum, double electron loss) to the cusp have been determined. For the ECC channel absolute doubly differential cross sections (DDCS) were also measured and compared to the corresponding DDCS values obtained with He + projectiles at the same experimental conditions. In another experiment an accurate measurement was performed for the shape of the ECC cusp by impact of goound-state and excited (metastable) He projectiles. The aim of this latter experiment was to check the validity of a model based on the formation of a low-lying virtual resonance state of the intermediate e − + He(2 1S) system. The case of the neutral hydrogen as projectile was also considered: the ECC cusp observed from 100 keV H 0 on He collisions is compared with the corresponding ECC cusp obtained with protons.