Measurements of Resonant Electron Capture inHe+on He Collisions

Abstract

Differential scattering of ${\mathrm{He}}^{+}$ ions by He atoms in the energy range of 0.4 to 25.0 keV has been studied as a function of scattering angle between 0.4 and 4.4\ifmmode^\circ\else\textdegree\fi{}. The scattered ion was analyzed to determine whether it had captured an electron after a single collision with a target atom. The limits in both energy and angle were chosen in order that a very wide variation in velocity and in impact parameter might be represented by the data. The high angular resolution needed in obtaining data at the small angles placed severe requirements on the beam-defining geometry and made precise determinations of the ion beam direction necessary. In addition, a spreading of the low-energy ion beam, which would have prevented taking data at small angles, was observed. The use of a special high-temperature collision chamber was necessary to prevent this spreading. Besides electron-capture effects, electron stripping and double capture were also studied, as represented by the ${\mathrm{He}}^{++}$ and ${\mathrm{He}}^{\ensuremath{-}}$ components among the scattered particles. For this ion-atom combination, electron capture by the ion is a result of resonant charge transfer and the electron capture probability is observed to oscillate when plotted versus incident ion energy. However, the electron-capture probability, which is found to be independent of scattering angle at the high ion energies, is observed to undergo a transition such that the electron-capture probability oscillates rapidly with angle, and is independent of ion energy at low energies. From these data, relationships among electron-capture probability, scattering angle, and ion energy are found and used to obtain an empirical equation that fits the data. This paper is the first of two consecutive papers, of which the second, by Everhart, analyzes these data in terms of the theory of resonant electron capture.