Absolute Experimental Cross Sections for the Ionization of Singly Charged Barium Ions by Electron Impact

Abstract

The absolute cross sections for the single ionization of ${\mathrm{Ba}}^{+}$ ions by electron impact have been measured as a function of incident electron energy over the electron energy range from below threshold (10.001 eV) to approximately 1000 eV. It is found that the cross section increases from 1.94\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ to 3.76\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ between 15.1 and 18 eV actual incident electron energy. This rapid rise is interpreted as the onset of autoionization. Some indication of structure occurring near the peak of the cross-section curve such as found in the isoelectronic system Cs is observed, but the relative magnitude of the apparent structure is of the same order as the 90% random-error confidence limits and thus cannot conclusively be regarded as being present. The maximum total error in the measurements is estimated to have its greatest value of less than \ifmmode\pm\else\textpm\fi{}20% at 15.5 eV while \ifmmode\pm\else\textpm\fi{}12% is typical of other energies. Of the total error, \ifmmode\pm\else\textpm\fi{}7% is deemed to be systematic. At incident electron energies below threshold, the cross section is found to be zero within 1% of the cross section at 48 eV. The measurements were performed in an all-metal ultrahigh-vacuum crossed-beam facility in which the nominal operating pressure was less than 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ Torr. The ion source was a water-cooled surface-ionization-type ion source while the electron source was a modified 6L6GC beam power tube. Continuous-beam techniques were used for the majority of the measurements, but modulated-beam methods were employed as a check. Measurements made by the two techniques agreed to well within the allowable experimental error and showed no systematic variations. Numerous consistency checks were performed to evaluate possible sources of experimental error such as pressure modulation of the background gas, focusing of the ion beam by the electron beam, and errors in the beam-profile determination. The present ${\mathrm{Ba}}^{+}$ ionization data are compared with the existing experimental and theoretical results.