In the last decade there has been a renaissance in laser spectroscopy at on-line facilities. This has included the introduction of ion traps and the use of laser ion sources to study the hyperfine structure of exotic nuclei far from stability and produce selective enhancement of isomeric beams. In-source spectroscopy has allowed the study of rare isotopes with yields as low as 0.1 atoms per second. In the case of high-resolution spectroscopy, cooling and trapping the ions has dramatically improved the sensitivity. Some elements that were previously inaccessible to laser spectroscopy are now available for study through the technique of in-trap optical pumping. This paper reviews the field of laser spectroscopy at on-line facilities, with an emphasis on new techniques. A summary of experimental data is presented.

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Collinear laser spectroscopy was performed on Ga (Z ¼ 31) isotopes at ISOLDE, CERN. A gas-filled linear Paul trap (ISCOOL) was used to extend measurements towards very neutron-rich isotopes (N ¼ 36-50 Nuclear structure has for some time been described by the single-particle (SP) states of nucleons in the shell model. The evolution and reordering of these levels along isotopic chains is explored at radioactive ion beam facilities to provide information on the nature of the nucleonnucleon interaction. Key to these studies is the determination of the value of the nuclear spin of each state, which provides a means of level identification. Whereas the spin may sometimes be inferred from nuclear decay and -spectroscopy data, laser spectroscopy [1,2] permits a measurement of the nuclear spin, in addition to the state's magnetic dipole and electric quadrupole moments. The latter two observables are very sensitive to the wave function and thus to the SP shell evolution. The sensitivity of the laser technique has been critically enhanced using bunched beams from a gas-filled linear rf quadrupole known as an ion beam cooler [3]. In this Letter we report the application of ISCOOL [4]-an ion beam cooler recently installed at ISOLDE-for collinear laser spectroscopy on Ga isotopes from stable to the magic N ¼ 50 shell gap, located 15 isotopes away from stability. For the first time g.s. spins have been measured, revealing sudden changes not observed in earlier experiments.The Ga isotopes have three protons outside the Z ¼ 28 shell gap. In a normal shell-model ordering, the three protons would occupy the p 3=2 level, leading to a g.s. spin I ¼ 3=2 for all odd-A Ga isotopes. However, in the Cu isotones with two protons fewer, it has been demonstrated that the proton SP ordering changes when neutrons start occupying the g 9=2 orbital around N ¼ 40 [5][6][7][8][9][10][11][12][13][14][15]. An inversion of the p 3=2 and f 5=2 SP levels was established recently in 75 Cu at N ¼ 46 [11], where the 5=2 À g.s. is near degenerate with a 3=2 À and 1=2 À state [11]. In this Letter we establish the g.s. spins and structure of the odd-A Ga isotopes from N ¼ 36 up to the N ¼ 50 shell closure, and we investigate the systematics of the 1=2 À , 3=2 À and 5=2 À levels.Fission fragments were produced in a thick UC x target (45 g=cm 2 ) using 1.4 GeV protons at an average current of $2 A. A proton-neutron converter [16] was used to suppress the Rb production. The Ga yield was selectively enhanced by a factor of 100 using the Resonant Ionization Laser-Ion Source [17], extracted and accelerated to 30 keV and mass selected. The ions were cooled and bunched by the newly-installed ISCOOL [4] and delivered to the collinear laser spectroscopy setup [18]. The ion beam was

Optical spectroscopy of a primordial isotope has traditionally formed the basis for understanding the atomic structure of an element. Such studies have been conducted for most elements and theoretical modelling can be performed to high precision, taking into account relativistic effects that scale approximately as the square of the atomic number. However, for the transfermium elements (those with atomic numbers greater than 100), the atomic structure is experimentally unknown. These radioactive elements are produced in nuclear fusion reactions at rates of only a few atoms per second at most and must be studied immediately following their production, which has so far precluded their optical spectroscopy. Here we report laser resonance ionization spectroscopy of nobelium (No; atomic number 102) in single-atom-at-a-time quantities, in which we identify the ground-state transition SP. By combining this result with data from an observed Rydberg series, we obtain an upper limit for the ionization potential of nobelium. These accurate results from direct laser excitations of outer-shell electrons cannot be achieved using state-of-the-art relativistic many-body calculations that include quantum electrodynamic effects, owing to large uncertainties in the modelled transition energies of the complex systems under consideration. Our work opens the door to high-precision measurements of various atomic and nuclear properties of elements heavier than nobelium, and motivates future theoretical work.