Measurement of the 1s2s2p 4P°5/2-1s2p3d 4F°9/2 electric-quadrupole transition in doubly excited N4+, O5+, F6+ and Ne7+ lithium-like ions

Doubly excited ions have been obtained in one-electron capture processes by metastable ions from Cs (6s) Incident heliumlike ions have been prepared in high metastable fractions by collision in a gas cell of hydrogenlike ions produced by an ECR ion source. The main interest of this work was to find the electric-quadrupole transition 1s2s2p 4P(5/2)(o) - 1s2p3d F-4(9/2)o among the doubly excited quartet and doublet systems of 3-electron ions. Although E2 transitions are in principle highly forbidden, the 2s P-4(5/2)o - 3d F-4(9/2)o transition has been measured in N4+, O5+, F6+ and Ne7+ by vuv spectroscopy taking advantage of non-negligible branching ratios and highly-state-selective-collisions.