Electron Correlation in the Formation of Hollow Li-Like Ions A. S. Alnaser 1,2 , A. Landers 1 , D. J. Pole 1 , S. Hossain 1 , E. P. Benis 2 , S. M. Ferguson 1 and J. A. Tanis 1 1 Physics Department, Western Michigan University, Kalamazoo, MI, 49008, U.S.A. 2 J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS, 66506, U.S.A. PACS Ref: 34.50.Fa Received July 15, 2003; accepted November 17, 2003 Abstract The role of electron correlation in the formation of double-K-shell vacancies in Li-like Be þ ; B 2þ ; C 3þ and O 5þ ions has been investigated using high-resolution zero-degree projectile spectroscopy. Using the collision velocity dependence for the cross sections of the observed doubly-K-shell vacant states, the contribu- tion of the e-e interaction was inferred and categorized in terms of shake and dielectronic models. The relative strength of the e-e interaction was examined as a function of the atomic number of the Li-like ion. 1. Introduction The excitation of a core (K-shell) electron by an incoming photon or ion can trigger a second core vacancy due to electronic rearrangement of the excited system, thereby producing a so-called ‘‘hollow’’ atom or ion, in which the innermost shell (the K-shell) is empty. The states of hollow lithium and lithium-like ions, where electron correlation effects lead to nln 0 l 0 n 00 l 00 ðn  2Þ states have recently been the subject of intense experimental and theoretical interest [1– 4]. Lithium, a four-body Coulombic system, provides an opportunity for investigating highly correlated multiply excited states when the K-shell is totally evacuated. In addition, in three-electron systems the 2s electron in the initial state allows both single-and double-K-shell vacancy production to be investigated from a single Auger emission spectrum, which is not possible for two-electron systems (except for double-K-shell excitation). Also, Li-like ions have the advantage that there are no long-lived metastable states that can complicate the interpretation of the observed spectra. For double-K-shell vacancy production in intermediate- to-high velocity atomic collisions where the collision time is small, the projectile interacts mainly with only one of the target electrons transferring it to an excited state or to the continuum to produce a K-shell vacancy. By subsequent rearrangement of the residual ion through the electron- electron interaction, the second electron may be excited or ejected. This process is referred to as TS1 (two-step with one projectile interaction [5]). At lower projectile velocities, the projectile may interact more strongly with the target electrons to independently produce two-K-shell vacancies. This process is referred to as TS2 (two-step with two projectile interactions). Electron correlation can be modeled in different ways depending on how the first K-shell electron is ejected. If the first electron is ejected slowly, it can interact with another electron through their mutual repulsion, such that sub- sequent excitation or ionization takes place and a hollow ion is produced. This model is referred to as dielectronic correlation [6]. On the other hand, if the first electron is ejected suddenly, thereby ‘‘freezing’’ the remaining elec- trons in their initial states, the wave function of the electrons in the residual ion must change and a second electron can be excited or ionized due to the subsequent electronic rearrangement. This mode is described in terms of shake dynamics. In the present work, single and double-K-shell vacancies produced in collisions of Li-like Be þ ; B 2þ ; C 3þ and O 5þ ions with a He target are investigated at intermediate-to- high velocity collisions by detecting Auger electrons emitted at 08 with respect to the beam direction using a tandem parallel-plate spectrometer. Using different Li-like ions allows for understanding of the variation of electron correlation as it reveals the role of the nuclear charge Z of the parent ion. Furthermore, the cross sections for producing the doubly-K-shell vacant states in the different Li-like ions are determined. The velocity dependence of these cross sections is used as a tool to help determine the mechanisms responsible for the hollow state formation in the Li-like isoelectronic sequence studied here, where this dependence exhibits different behaviors depending on whether the double-K-shell vacancy is produced by the TS1 or TS2 mechanism. 2. Single K-shell excitation The present work has been conducted at Western Michigan University. Li-like ions were accelerated to intermediate- to-high velocities before they collided with a neutral helium target. In this collision velocity range, the plane-wave Born approximation (PWBA) can be used to calculate the cross sections for the single-K-shell excitation in the Li-like ions by impact of the helium nucleus. Figure 1 shows the measured and calculated K-shell excitation cross sections for the 1s2s2p 2 Pð1s ! 2pÞ and 1s2s 22 Sð1s ! 2sÞ states in C 3þ ions, as functions of the collision velocity v: The calculated cross sections were evaluated using a PWBA code [7] for proton impact and scaled using Z p ¼ 2 for the helium nuclear charge. In the case of the measured values, the cross sections for 1sð2s2p 3 PÞ 2 P and 1sð2s2p 1 PÞ 2 P have been added in order to compare with theory. The measured cross sections for the 1s2s2p 2 P state vary with the predicted ðlnðvÞ=v 2 Þ dependence of the Born approximation for these allowed (dipole) transitions, while for 1s2s 22 S the measured cross sections vary as 1=v 2 ; also as predicted by the Born approximation for these forbidden (monople) transitions [8,9]. The good agreement between the measured cross sections and the Born calculations show that perturbation Physica Scripta. Vol. T110, 137–140, 2004 # Physica Scripta 2004 Physica Scripta T110