Electron-impact ionization ofLaq+ions (q=1,2,3)

Experimental cross sections for single ionization of ${\mathrm{La}}^{+}$, ${\mathrm{La}}^{2+}$, and ${\mathrm{La}}^{3+}$ ions as well as for double ionization of ${\mathrm{La}}^{+}$ and ${\mathrm{La}}^{2+}$ ions and for triple ionization of ${\mathrm{La}}^{2+}$ ions are presented in an electron-impact energy range from threshold up to 1000 eV. By using a fast energy-scanning technique with step widths of about 40 meV and an energy resolution of 0.4 eV, fine details in the cross sections and especially resonant contributions could be observed. Resonant excitation of ${\mathrm{La}}^{2+}$ at 95 eV with subsequent emission of three electrons appears in our interpretation to account for 30% of the total measured double-ionization cross section. The data are interpreted on the basis of calculations of energy levels and distorted-wave calculations of direct ionization from various electron subshells and of excitation-autoionization processes. A detailed analysis of the ${\mathrm{La}}^{3+}$ single ionization both for ground-state and 5$p^5$4f metastable ions is presented.