On page 1026, the last sentence of the second paragraph should read, "However, Lablanquie et al.[5] could show from their ͑g, 2e͒ data that at E exc 4 eV the ratio K 0 (which is proportional to the a u term in their notation) to K 1 is small, typically down to 1͞19."

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Cold target recoil ion momentum spectroscopy has been used to image the momentum distributions of continuum electrons liberated in the impact of slow He 1 and He 21 ions on He. The distributions were measured for fully determined vector impact parameter. The spectra show that the electron momenta lie mainly in the collision plane and display a structure which strongly suggests that it is p states which are dominantly promoted into the continuum in such collisions, but that interfering s and perhaps d amplitudes are also important. [S0031-9007(98)07407-9]

We have measured the distribution of photoion charge states resulting from vacancy cascades and electron shakeoff following AT-shell photoionization of Ar 2+ ions. The Ar 2+ ions were produced by electron-impact ionization of Ar, stored in a Penning ion trap, and then photoionized using broadband synchrotron radiation. Time-reversed integration of coupled differential equations were used to account for contributions from electron capture with residual gas. The results are in reasonable agreement with a simple theoretical prediction.PACS numbers: 32.80.Fb, 32.80.Hd, 34.50.Fa We have studied the AT-shell photoionization of a stored Ar 2+ target and the L-shell photoionization of a cold ion gas l consisting initially of Xe ions with a range of charge states from 2+ to 11+. The experimental determination of the distribution of photoion charge states from the Ar 2+ measurements is described here. The Xe results, which gave a qualitative indication of the sequential photoionization of ions, are described elsewhere. 2 These are the inaugural measurements on the inner-shell photoionization of multiply charged ions using synchrotron radiation (SR).The present measurements have both fundamental and practical importance. In a recent review, 3 Manson emphasized that even broad theoretical predictions of the charge-state dependence of inner-shell photoionization are untested, and that experiment is needed to put theory on a firmer footing. Calculations of the distribution of charge states resulting from inner-shell photoionization are also an active theoretical area. 4,5 Charge-changing collisions of low-energy, multiply charged ions are important in laboratory and astrophysical plasmas, 6 and have consequences associated with photoionization by Compton-scattered y rays in the ejecta of supernova SN 1987A. 7 On the experimental side, these measurements provide direct evidence of the feasibility of potential inner-shell photoionization studies at the next generation of hard-x-ray light sources. 8 Earlier pioneering studies have been reported for the outer-shell photoionization of singly charged ions 9 in a beam merged with monochromatized UV synchrotron radiation, and photoionization of light ions has been studied using radiation from a laser-produced plasma. 10 Near their respective thresholds, photoionization cross sections are of order 10~1 9 cm 2 for the K shell of argon and its ions but 10 ~1 5 cm 2 for the outer shells of many atomic and ionic systems. 9 Inner-shell photoionization measurements thus require not only higher-energy photons to excite more-tightly-bound electrons, but also higher target densities and/or greater photon fluxes to compensate for the much lower cross sections. As proposed earlier, 11,12 these difficulties were overcome by using focused bending-magnet SR from the x-ray ring of the National Synchrotron Light Source (NSLS), and by utilizing ion-trapping techniques to produce a stored multiply charged ion target.The apparatus and the basic experimental approach to the SR production and storage of...