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AbstracL Resonance ionization spectroscopy in collinear geometly has been successfully applied to a fast beam of ytterbium atoms. The atoms were excited stepwise into a Rydberg state by pulsed laser lighl, ionized in an electrical field and deflected onlo a secondaty electron detector. The efficiency was 1 x lo-' delecled ions per incoming atom an a background from collisional ionization of 1 x 10V8. The technique has been exploited for 1he measurement of hyperfine structures and isotope shifls of unstable ytterbium isotopes, in particular IS7Yb, lS9Yb and 17rYb.

The nuclear magnetic moments of 213 Ra and 225 Ra have been measured at ISOLDE (isotope separator at the CERN synchrocyclotron) by observation of the Larmor precession of optically pumped atoms in a fast beam. The results ^/( 213 Ra) =0.6133(18)//^ and ^/( 225 Ra) = -0.7338(15)^* provide an accurate test of ab initio and semiempirical calculations from optical hyperfine structures in Ral and Rail. PACS numbers: 21.10.Ky, 27.80,+w, 27.90.+b, 32.80.Bx Over the past few years collinear fast-beam-laser spectroscopy on line with accelerators has proved to be a very fruitful technique for the study of exotic very unstable nuclei. x Nuclear magnetic moments, for instance, are deduced from hyperfine-structure (hfs) measurements through the magnetic interaction constant A and the nuclear spin /. This requires of course, the knowledge either of the magnetic field H e {0)-AIJ/iu(1) created by the valence electrons at the nucleus, or of the nuclear magnetic moment of at least one isotope. Moreover, this procedure implies that the hyperfine anomaly is negligible, and consequently the uncertainty on the nuclear results is rather large (1% typically) even if the hfs constants are known to a much better precision. Such measurements in a series of Ra isotopes were reported three years ago. 2 Since no direct measurement of the nuclear magnetic moment had been performed previously for any of the Ra isotopes, it was necessary to evaluate H e (0), and this is the main source of the estimated ± 5% error in the /ij values. There have been attempts to reduce this error by both theoretical ab initio calculations 3 " 5 and improved semiempirical analyses. 6 We report here the first direct measurement of the nuclear magnetic moments of 213225 Ra which eliminates the uncertainty about H e (0) in the determination of the Hi values for the whole isotopic sequence and also tests the atomic hfs theory.The experimental method consists of our measuring the Larmor precession of the ground-state magnetic moment in an external static magnetic field by observing the fluorescence of a fast atomic beam interacting with a collinear resonant laser beam. Such experiments were first performed with paramagnetic (J**0) neutral atoms 7 or ions. 8 When the experiment is performed with diamagnetic atoms (/-0 ground state), the gyromagnetic factor gj and thus the nuclear magnetic moment fii can be deduced directly from the precession frequency.The principle of the method has been described in a previous paper devoted to a pilot experiment on Ba isotopes 9 : The fast neutral atoms are first aligned (or oriented) by optical pumping through linearly (or circularly) polarized laser excitation (zone A in Fig. 1). Then, these aligned (or oriented) atoms precess at the Larmor frequency in a strong external magnetic field (zone B). In this region, the atoms are out of resonance with the laser because of the large Zeeman splitting of the upper level of the transition. At the exit of the magnet gap the fast atoms are reexcited by the laser light and the fluorescence ...

We present here an extension to low-lying states of francium (lOs, 1 Is, 8d, 9d) of a previously published high-resolution experiment performed on the ns and nd Rydberg levels. These four levels were investigated at CERN with the on-line mass separator ISOLDE using stepwise laser excitation in collinear geometry. Accurate energy measurements lead to a more reliable determination of the quantum defects of the whole ns and nd series, together with the ionization potential for 2'2Fr (nuclear spin I = 5 ) which is found to be 32 848.872 (9) cm-'. The measurement of the fine structure splittings for the nd 2D3,2,5,2 doublets has been completed at the same time. The main result of this study is the first observation and measurement of hyperfine structures in ' S and 'D states. Dipolar hyperfine constants deduced from hyperfine splittings are as follows: A(102S,,,) = 401 (5) MHz; A(S2D,,,) = -7.2 ( 6 ) MHz and A(9,DS,,) = -3.6 (4) MHz. The negative sign of the A(n2Dslz)/A(n2D5,?) ratios is a clear indication of the large core polarization effects as already pointed out in the case of other heavy alkali elements. A survey of all the available energy terms is also given to summarize the current knowledge of the francium optical spectrum.A(112S,,2) =225 ( 3 ) MHz;A(S2D3,2) = 13.0 (6) MHz; A(92D3,2) ~7 . 1 (7) MHz;

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Recently isotope shifts of 72, Kr and 77-100 Sr have been measured at the ISOLDE/ CERN mass separator facility by collinear laser spectroscopy. The deduced changes in mean square charge radii reveal sharp transitions in nuclear shape from spherical near the magic neutron number N=50 towards strongly deformed for both the neutron deficient and neutron rich isotopes far from stability. The mean square charge radii of the neutron deficient isotopes exhibit a sign change of the odd-even staggering (OES), i.e. below the neutron number N=46 the radius is systematically larger for the odd-N nuclei than for their even-N neighbours. This is in contrast to the situation of normal OES which is observed for the heavier isotopes. The inversion of the OES is interpreted as an effect of polarization, triggered by the addition of an unpaired neutron and driving the soft eveneven core into stable strong deformation. accepted for publication in Europhysics Letters (IS80 and IS304)

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