Single-electron-capture cross sections for 1-10-keV Li+ ions in alkaline-earth vapors.

Abstract

Total charge-neutralization cross sections have been measured with use of an attenuation method for ${\mathrm{Li}}^{+}$ ions in the energy range from 1 to 10 keV incident on thin targets of Mg, Ca, Sr, and Ba vapors. These cross sections exhibit an energy dependence characteristic of asymmetric charge exchange, increasing with collision energy up to a broad maximum, then decreasing at higher energies. The ${\mathrm{Li}}^{+}$+Ba cross section reaches a maximum value (${\ensuremath{\sigma}}_{m}$) of 4.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}14}$ ${\mathrm{cm}}^{2}$ at \ensuremath{\sim}3.5 keV impact energy. The corresponding values of ${\ensuremath{\sigma}}_{m}$ for Sr and Ca are 2.85\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}14}$ ${\mathrm{cm}}^{2}$ at 6.0 keV, and 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$ at 10 keV. The ${\mathrm{Li}}^{+}$+Mg cross section only attains a value of 1.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$ at the highest energy measured (10 keV), thus, ${\ensuremath{\sigma}}_{m}$ clearly occurs at even higher energies for this system. The energies at which these cross-section maxima occur increase monotonically through the series from Mg to Ba, while the magnitude of ${\ensuremath{\sigma}}_{m}$ decreases in that order. This ordering is consistent with the inverse ordering of the asymptotic energy defects for the process: ${\mathrm{Li}}^{+}$(1${\mathrm{s}}^{2}$)+M(${\mathrm{ns}}^{2}$)\ensuremath{\rightarrow}Li(2s) +${M}^{+}$(ns), where M is an alkaline-earth element.