Inner-shell photodetachment of Li^-

Abstract

Absolute cross section for photodetachment of Li − ions near the 1s threshold leading to Li + formation has been measured by overlapping a beam of Li − ions with synchrotron radiation from an undulator. Multiple excitation to the coreexcited Li 1s(2s2p 3 P) 2 P and Li 1s(2p3s 3 P) 2 P states, i.e. conjugate shakeup, is found to be much stronger than observed for any neutral atom or positive ion, reflecting the dominant influence of electron correlation for negative ions. The cross section for multiple excitation is significantly larger than for single excitation.Negative ions represent a special class of atomic systems with properties markedly different from neutral atoms and positive ions. In recent years, weakly bound negative atomic ions have attracted considerable interest, and such ions are of fundamental importance in atomic physics. Their structure and dynamics form a critical test for theoretical calculations since the additional electron is bound by means of electron correlation. Recently theoretical and experimental studies of atomic negative ions have been reviewed focusing on their binding energies (Andersen et al 1999), resonances (Buckmann and Clark 1994), and photodetachment cross sections (Ivanov 1999). The experimental photodetachment studies have only been concerned with photodetachment of outer-shell electrons, with lasers being used as the photon source, whereas experimental cross sections for photodetachment of inner-shell electrons have never been reported. The reason may be a combination of the very low densities which can be obtained for negative atomic ions, the limiting photon flux available from bending magnets at synchrotron-radiation facilities and relatively small cross sections. However, with access to synchrotron-radiation beam lines equipped with insertion devices, the increased photon flux allows inner-shell photodetachment studies of atomic negative ions to be performed.This experimental study was initiated with the aim of exploring the competition between photoinduced single and multiple excitation of a weakly bound negative ion exhibiting strong correlation in the ground state. The negative ions of the alkali metals or the alkaline earth metals are suitable candidates for such an investigation (see Andersen et al 1999). We have chosen the Li − system due to its simplicity as a closed-shell ion with only 4 electrons and because the binding energy of its 1s electrons is convinient for the undulator beamline of the ASTRID storage ring. Li − is a highly correlated system-without introducing electron correlation the system is not bound. Thus, correlation is not a small perturbation on the system, rather a dominant factor of the behaviour of the system.Following the negative hydrogen ion, the negative lithium ion is the most important closedshell negative ion which is simple enough to be theoretically tractable in a rigorous manner.

Prominent role of multielectron processes inK-shell double and triple photodetachment of oxygen anions

“…Apart from the double photodetachment, that was considered for other light ions in previous studies [7,8,[10][11][12][13], we here also report the triple photodetachment in the photon energy range of K-shell excitation and ionization. Two K-shell ionization thresholds were identified and the corresponding threshold energies were determined.…”

“…The low-excitation levels of negative ions are readily accessible by laser spectroscopy (see, e.g., [3][4][5][6]). Therefore, this technique has been a prime source of experimental information about the mutual interactions among the valence electrons.A sensitive tool for studying the interactions between the valence and the core electrons is inner-shell ionization of negative ions [7,8]. Here, we apply the photon-ion merged-beams technique (see [9] for a recent overview) to determine the absolute cross sections for double and triple ionization (detachment) of oxygen anions in the photon energy range 524-543 eV.…”

“…To test and better understand the theoretical prediction of such cascades, we performed extremely comprehensive quantum calculations for the formation of the intermediate resonances and for the complex deexcitation pathways at a level of detail never attempted before. In particular, all resonance parameters (resonance energy, natural line width, and strength) of the 1s 2s 2 2p 6 photoionization resonance as well as the (absolute) cross sections and ion yields are determined independently by experiment and multi-configuration Dirac-Fock (MCDF) calculations.Previous experimental studies of double detachment via inner K-shell excitation/ionization were performed for several anions lighter than O − , i.e., for Li − [7,8], B − [10], and C − [11,12]. A recent review of related computations is provided in Ref.…”

“…A sensitive tool for studying the interactions between the valence and the core electrons is inner-shell ionization of negative ions [7,8]. Here, we apply the photon-ion merged-beams technique (see [9] for a recent overview) to determine the absolute cross sections for double and triple ionization (detachment) of oxygen anions in the photon energy range 524-543 eV.…”

“…Previous experimental studies of double detachment via inner K-shell excitation/ionization were performed for several anions lighter than O − , i.e., for Li − [7,8], B − [10], and C − [11,12]. A recent review of related computations is provided in Ref.…”