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Abstract. The frequencies of the 2S-3S two-photon transition for the stable lithium isotopes were measured by cavity-enhanced Doppler-free laser excitation that was controlled by a femtosecond frequency comb. The resulting values of 815 618 181.57(18) and 815 606 727.59(18) MHz, respectively, for 7 Li and 6 Li are in agreement with previous measurements but are more accurate by an order of magnitude. There is still a discrepancy of about 11.6 and 10.6 MHz from the latest theoretical values. This is comparable to the uncertainty in the theoretical calculations, while uncertainty in our experimental values is more than a hundred-fold smaller. More accurate theoretical calculation of the transition frequencies would allow extraction of the absolute charge radii for these stable isotopes, which in turn could improve nuclear charge radii values for the unstable lithium isotopes.

Abstract. We have performed isotope shift measurements in the 2s 1/2 → 2p 3/2 transition of Be + ions using advanced collinear laser spectroscopy with two counterpropagating laser beams. Measurements involving a frequency comb for laser stabilization and absolute frequency determination allowed us to determine the isotope shifts with an accuracy of 2 MHz. From the isotope shifts between 9 Be and 7,10,11 Be, high-accuracy mass shift calculations and the charge radius of the reference isotope 9 Be we determined nuclear charge radii for the isotopes 7,10 Be and the one-neutron halo nucleus 11 Be. The results are compared to nuclear-structure calculations using the Fermionic Molecular Dynamics model which reproduce well the general trend of the radii. Decreasing charge radii from 7 Be to 10 Be are explained by the cluster structure of the nuclei. The increase from 10 Be to 11 Be is mainly caused by the halo neutron by which the 10 Be core moves relative to the center of mass. Polarization of the 10 Be core has only a small influence on the charge radius.

Absolute transition frequencies of the 2s 2 S1 /2 → 2p 2 P1 /2, 3 /2 transitions in Be + were measured with a frequency comb in stable and short-lived isotopes at ISOLDE (CERN) using collinear laser spectroscopy. Quasi-simultaneous measurements in copropagating and counterpropagating geometry were performed to become independent from acceleration voltage determinations for Doppler-shift corrections of the fast ion beam. Isotope shifts and fine structure splittings were obtained from the absolute transition frequencies with accuracies better than 1 MHz and led to a precise determination of the nuclear charge radii of 7,10−12 Be relative to the stable isotope 9 Be. Moreover, an accurate determination of the 2p fine structure splitting allowed a test of high-precision bound-state QED calculations in the three-electron system. Here, we describe the laser spectroscopic method in detail, including several tests that were carried out to determine or estimate systematic uncertainties. Final values from two experimental runs at ISOLDE are presented and the results are discussed.

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