Charge Transfer Between Li Ions and Li Atoms in the 14-1000-eV Energy Region

Abstract

By means of a crossed-beam technique, the absolute cross section for charge transfer between lithium ions and lithium atoms has been measured for incident ion energies ranging from 14 to 1000 eV. The lithium ions are produced thermally in a $\ensuremath{\beta}$-eucryptite source and are accelerated and focused into a ribbon beam in a Pierce gun. The total energy spread of the beam is less than 3% at all energies; mass analysis shows that above 1000\ifmmode^\circ\else\textdegree\fi{}C emitter temperature the impurity ions in the beam are below 0.2%. This ion beam traverses a high-density, collimated lithium vapor beam issuing from an oven. A typical density of neutral lithium atoms in the interaction region is ${10}^{13}$ atoms/${\mathrm{cm}}^{3}$. The slow ions produced in the charge-transfer interaction are collected on a negatively biased cage surrounding the interaction volume of the two beams. The slow-ion charge-transfer current completely accounts for the ion-beam attenuation, thus indicating that all scattered beam ions are collected. The vapor-beam density was determined by condensing the vapor on a cold target, dissolving the metallic lithium thus collected, and titrating the resulting solution. The cross section for charge transfer varies from 240\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at 14 eV incident ion energy to 80\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at 1000 eV, and has the expected energy dependence for resonant charge transfer ${Q}^{\frac{1}{2}}=19.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\ensuremath{-}3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}logE,$ where the incident ion energy $E$ is in eV and $Q$ is in ${\mathrm{cm}}^{2}$.