Triple Ionization of Lithium by Electron Impact

Ejection of the three electrons from lithium in a single electron collision has been observed for the first time. Triply charged lithium was observed in an ion time-of-flight spectrum following electron impact on a sample of ultracold, trapped lithium. The higher signal/background afforded by the trap environment made the observation of ${\mathrm{L}\mathrm{i}}^{3+}$ possible. We measured the ratios of triple-to-double and double-to-single ionization at an impact energy of 1000 eV. The $3+/2+$ ratio is $\approx 0.001$, a value 2 orders of magnitude lower than semiempirical predictions. We present a simple method that uses photoionization data combined with sum-rule analysis to predict the asymptotic charge-state ratios. The sum-rule predictions compare reasonably with experiment and shake calculations, but disagree sharply with the semiempirical estimates.

Figure 1

Top panel shows the cumulative ion time-of-flight spectrum for ${\mathrm{L}\mathrm{i}}^{3+}$ and ${\mathrm{L}\mathrm{i}}^{2+}$. Bottom panel shows a typical spectrum including the ${\mathrm{L}\mathrm{i}}^{2+}$ and ${\mathrm{L}\mathrm{i}}^{1+}$ peaks.