Absolute Cross Sections for Single Ionization of Alkali Ions by Electron Impact. I. Description of Apparatus andLi+Results

The absolute cross sections for the single ionization of ${\mathrm{Li}}^{+}$ ions by electron impact have been measured over the electron-energy range from below threshold (75.6 eV) to 800 eV. The measurements were performed with an ultrahigh-vacuum crossed-beam facility operating under continuous, rather than the usual time modulated, beam conditions. A detailed description of the apparatus and experimental method is presented. Numerous checks were performed to justify the use of the continuous-beam measurement technique developed for this experiment, particular attention being paid to beam intensities, beam profiles, space charge, signal-to-noise ratio, and ion-beam composition. The error in the measured cross sections is believed not to exceed ±12% above 150 eV electron energy; it may be as large as ±21% at 90 eV. Of this possible error, an amount ±6% is considered systematic.