Measurements of the electron-impact double-to-single ionization ratio using trapped lithium

The ${\mathrm{Li}}^{2+}$ to ${\mathrm{Li}}^{+}$ production cross-section ratio of ground-state atomic Li by electron-impact ionization has been measured for electron energies ranging from 200 eV to 1500 eV. The measurements were done using a pulsed, ion imaging time-of-flight spectrometer with Li atoms confined in a magneto-optical trap. The ratios are more accurate than the single earlier result for the ${\mathrm{Li}}^{2+}$ to ${\mathrm{Li}}^{+}$ ratios, a composite of two absolute measurements, and are systematically lower. Both experiments show similar energy dependences that disagree with the trend predicted by a semiempirical formulation. These measurements provide a benchmark for theoretical studies of electron-impact double ionization.