We report the first measurement of the ratio of double-to-single photoionization of helium well above the double-ionization threshold. Using a time-of-flight technique, we find He"*""""/He"*" = 1.6%±0.3% at /zv==2.8 keV. This value lies between calculations by Amusia (2.3%) and by Samson, who predicts 1.2% by analogy with electron-impact ionization cross sections of singly charged ions. Good agreement is obtained with older shake calculations of Byron and Joachain, and of Aberg, who predict 1.7%.

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...