No abstract

Electron-impact ionization cross sections for He-like Li + ions have been measured with extreme precision (0.01% statistics). Thus, two new mechanisms in the ionization of positive ions both involving two-electron excitations could be quantitatively investigated. We see (1) direct excitation of both K electrons into the L shell with subsequent autoionization and (2) resonant capture of the incident electron by the ion with correlated KK-LL excitation of both core electrons leading to a triply excited Li atom which then decays by the emission of two correlated electrons. Strong interference of process (2) with direct ionization is observed.PACS numbers: 34.80. Dp, 34.80.Kw, 52.20.Fs Correlated multielectron transitions in atoms in which two or more electrons change their quantum numbers had been considered already in the early days of atomic quantum theory. l Two-electron excitations were first observed in low-energy electron scattering experiments. 2 Experimental evidence for correlated two-electron deexcitation into a doubly ionized inner shell with emission of a single photon 3 or electron 4 was found nearly fifty years later. Another correlated multielectron transition is the ejection of two outer electrons due to a one-electron jump into an inner-shell vacancy 5 ' 6 which is often called auto-double ionization (or double-Auger process).Interpretation and quantitative analysis of the first observations of multielectron phenomena were difficult. Some of the complications can be avoided by studying the simplest atomic systems with the minimum number of electrons required for the correlated transitions mentioned above, namely two-and three-electron systems, respectively. Thus, triply excited configurations could be associated with structure in the high-energy electron spectra of foil-excited Li atoms. 7 Even in electron scattering from He triply excited resonances had been found 8 and were attributed to (2s 2 2p) 2 P and (2s2p 2 ) 2 D states in the He" ion. 9 These resonances were confirmed in a number of further experiments, 10 one of which used the detection of He + ions produced by the decay of these resonances. 11 Similarly, structure in the cross section for detachment from H ~ by electron impact was observed 12,13 and interpreted as a result of the formation of short-lived states in the H ~~ ~ ion. 14,15 In this Letter we report the first observation of direct two-electron excitation contributing to electron-impact ionization of positive ions. Furthermore, we identify contributions from resonantly formed triply excited states which both are populated and decay by correlated two-electron transitions. In particular, we observe features in the ionization of Li + arising from intermediate two-and three-electron states with two vacancies in the K shell. Nonresonant double excitation with subsequent autoionization e + Li + (l* 2 )-[Li + (2/2/')]**+e-Li 2+ (U)+e + e (1) was found at electron energies above a threshold of 146 eV. Below this energy we observed resonant processes involving correlated double excita...

No abstract