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.

The absolute single-and double-photoionization cross sections of singly charged Fe ions have been measured from 15.8 to 180 eV using the merged-beam technique. The data yield information about the photoionization continua and the resonance structures resulting from excitation of the outer 3d and 4s electrons as well as the inner 3p and 3s electrons. The vast majority of the Fe+ target ions were present in the ground-state configuration, 3d64s, and term,6D. The experimental data have been compared with several calculations, for example R-matrix calculations from the Opacity Project and data obtained using the central-field approximations. The experimental data are available at http://www.ifa.au.dk/amo/atomphys/atomphys.htm.

The absolute cross section for photoionization of Cr+ has been measured using the merged-beam technique, focusing on the region of the ‘giant’ resonance. These new absolute data differ considerably from previous experimental data, with the result that the agreement between experiment and existing calculations has improved substantially. The neutral and ionized Cr spectra, previously believed to be completely different, are shown here to be rather similar. The role of the 3d5(6S) state in determining the special position of Cr among the 3d elements is emphasized. The experimental data are available at http://www.phys.au.dk/amo/atomphys/atomphys.htm.

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The absolute cross section for photoionization of Mn+ has been measured using the merged-beam technique, focusing on the region of the ‘giant’ 3p → 3d resonance. The main discrepancy between theory and earlier experiments has been removed. However, more extensive relativistic calculations are required for a rigorous comparison with theory.