Electron-impact single-ionization of singly and multiply charged tungsten ions

Employing the 'crossed-beams' technique, electron-impact ionization cross sections for the single-ionization of W q+ ions in charge states q=1...10 have been measured. The cross sections for the ionization of ions in charge states q>or=5 show significant contributions below the ground-state threshold caused by the ionization of ions in excited, long-lived metastable states in the parent ion beam. In the case of W 6+ , the measured cross section is completely dominated by metastable contributions. For ions in charge states q=3...7, excitation-autoionization from the ground state as well as from excited, metastable states contributes strongly to the cross sections. This results in a strong underestimation of the measured cross sections by the semiempirical Lotz formula in the energy range between the ionization threshold and the cross section maximum.