The capture of many electrons by Ar !7+ ions, at low velocity, near a metallic surface, has been studied. Multiexcited bound states with many electrons in the outermost shells (hollow atoms) have been observed. The surrounding of an ionic excited core by many outermost electrons greatly decreases the lifetimes of the states. This characteristic decrease explains the main striking features of the relaxation of the ions.

The elastic scattering of~particles near 180' was mea8»red in the vicinity of the Coulomb blrrjer for 110 112 & 114, 118Cd 112~114~118~118~120~122~124Sn 122~14, 128~128~1 30Te 144~148~iw, 152S and 204~2 08Pb. An optical-model analysis using Moods-Saxon potentials shows that the usual parameters -of the real part of the potential V R p& and a must obey the relationship V exp[pppf R0 2)/a] =0.2 MeV in order to fit the data. The~-nuc1eus distance R0 2 at which the nuclear potential depth is -0.2 MeV can then be determined for each nucleus within + 0.03 fm. An analysis in terms of a folding model was performed for 0 Pb and 4Te. For the class of potentials thus obtained, it is the o -nucleus distance at 0.5 MeV depth rather than at 0.2 MeV that appears to be best determined. The same. analysis determines the radius RFD-0 002 at which the nucleon density is 2x10 nucleon/fm . The value of RFD is found to depend mostly on the m-nucleon effective interaction used, and very little on the functional form of the density distribution. Further evidence is presented in favor of the Gaussian interaction -U0 exp(-E~2) with U0= 127 MeV and%=0. 6 fm, which has been proposed by Sumner and which leads to the probable value RFD=R0 2 -{3. 06+ 0.03) fm. Other interactions are not excluded, however, and considering those proposed so far in the literature leads to RFD=R0 2 -(3.11+0.14) fm. The average variation of R Fp with mass number is found to be (R&D) =(1. 355A +0.87) fm for spherical nuclei. The rate of variation. of |'RFD) with mass number is found to be in good agreement with the droplet model predictions, which is taken as an evidence that the surface thickness of spherical nuclei is practically constant from Sn to Pb.

The hyperfine-quenched transition 2 3 /Vl 'So has been observed in heliumlike gadolinium (Gd 62+ ) in the two isotopes ,55 Gd and ,57 Gd. The lifetime for the transition (to) has been measured for each isotope and found to be r 0 (l55) = 13.43(27)x 10" l2 sec and r 0 (l57) =7.65(55)x 10" ,2 sec. From the measured lifetimes a value is inferred for the absolute value of the 2 3 Po-2 3 P\ fine-structuring splitting |A£o-i| in Gd 62+ with the result |AEo-i| = 18.57(19) eV, where the error represents 1 standard deviation. This result is compared with calculations based on the multiconfiguration Dirac-Fock method and on the unified method. PACS numbers: 32.30.Rj, 12.20.Fv, 31.30.Jv, 32.70.FwThe splittings of the I s Pj levels [l] in two-electron ions (AEJ.J) are determined largely by the electron-electron interaction. The measurement of these splittings constitutes, therefore, a test of theories of this interaction in the simplest atomic system in which it can be observed. Because of the rapid scaling of the relativistic part of this interaction with atomic number Z, measurements at high Z provide a particularly sensitive test of the relativistic theory. Calculations of the fine-structure splittings generally include the effects of relativity by a perturbation expansion in the parameter (Z«) 2 . The problems associated with this approach at high Z are illustrated by the fact that in ordinary helium (Z=2) the calculations [2] are accurate at the ppm level, whereas the most elaborate calculations [3,4] at Z=64 are accurate at only the few percent level. Accurate experimental measurements performed at high Z can therefore act to stimulate further theoretical work in including relativistic effects in the calculation of two-electron energy levels.Accurate experimental measurements of the two-electron fine-structure splittings in the I s Pj multiplet have been performed in He and Li + using radio-frequency spectroscopy [5] and laser spectroscopy [6]. The extension of laser spectroscopy to the heliumlike ion F 7+ was achieved by Myers et al.[7], who measured the interval 2 3 / > i-2 3 / > 2 (AE1.2). However, extension of the laser method to much higher Z is experimentally not feasible. Indirect measurements using x-ray and UV spectroscopy have been performed and are summarized in review articles by Martin [8] and Desequelles [9], but such measurements have not been very precise. So far there have been no measurements reported with sufficient precision to examine the theoretical predictions at high Z. In thisLetter we describe the first measurement at high Z with a precision smaller than the current theoretical accuracy.The measurement reported here is of the interval AEo.\ in the two-electron ion Gd 62+ . Gadolinium has a number of stable even-even isotopes with nuclear spin 7=0. For these isotopes, the radiative decay 2 3 /Vl '-So is strictly forbidden and the 2 3 / > o state undergoes E\ decay to the 2 3 S1 state with a calculated lifetime [10] of r(2 3 / > 0 ) =0.339xl0~9 sec. Gadolinium also has two stable isotopes ( ,55 ...