Double-K-shell-vacancy production in Li by 95 MeV͞nucleon Ar 181 projectiles was investigated using high-resolution electron spectroscopy. The two K vacancies are found to come about mainly by ionization and excitation, giving rise to excited states in hollow Li 1 . Strong line intensities from the 2s 2 1 S and 2s3s 3 S configurations provide spectral identification for the electron-electron interaction. Production of the 2s3s 3 S state, with an intensity greater than that for 2s 2 1 S, is attributed to a threeelectron transition involving correlation. PACS numbers: 34.50.Fa, 32.80.Dz Multielectron transitions in atoms provide insight into the fundamental nature of atomic structure by probing dynamical electron correlation effects. In recent years, the importance of the electron-electron ͑e-e͒ interaction in understanding the multiple excitation or ionization of atoms has been widely recognized [1][2][3][4][5][6].In ion-atom collisions, multielectron transitions can result from the e-e interaction or from the nucleuselectron ͑n-e͒ interaction (n here not to be confused with neutron). If a multielectron transition leads to an empty K shell, then a so-called "hollow atom" (or ion) is produced. Such double-K-shell vacancy production in a target atom by ion impact can be caused by separate n-e interactions, or by an n-e interaction followed by an e-e interaction. In the former case, the process is referred to as TS2 (two-step with two projectile interactions), and in the latter case it is called TS1 (two-step with one projectile interaction), a process which implies dynamic electron correlation [4,6]. Such dynamic correlation is well known in photoionization [2,7], where multielectron transitions resulting from a single photon impact can be caused only by the e-e interaction. Thus, hollow-atom production by photon impact corresponds to TS1 in the case of ion impact. During the past decade, double-K-shell ionization of He by fast ions and photons has attracted much interest [8] due to the insight it provides into dynamic correlation effects.Dynamic electron correlation generally falls into two categories, corresponding to whether the first electron is emitted slowly or suddenly. For slow emission, subsequent excitation or ionization of a second electron involves the mutual scattering of two electrons, i.e., it is dielectronic in nature [4]. On the other hand, sudden emission can result in a subsequent electron transition due to the change in potential seen by the second active electron as the excited system relaxes [2]. This latter type of transition is a "mean-field" effect referred to as a shake process.To investigate dynamic correlation in atomic collisions, we consider the formation of hollow Li caused by fast projectiles. Early observations of hollow Li with incident ions of intermediate energy were made by Ziem et al.
The energy distributions of H + fragments produced in 345 keV Xe 23+ + H 2 and 75 keV O 5+ + H 2 collisions have been investigated experimentally as a function of the detection angle. For both systems, the experiment shows strong deviations from the molecular Coulomb explosion. Two-and three-body model calculations have also been performed to understand the energy spectra. In the collisions involving the heaviest projectile Xe 23+ , the two-body calculations reproduce the experimental data satisfactorily, indicating that the recoil energy transferred to the target plays an important role in the fragment-energy distributions. On the other hand, for the system O 5+ +H 2 , the deviations between calculations and experiment suggest that the interaction between the outgoing projectile and each fragment plays the major role.
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