Isotope-shift measurements of stable and short-lived lithium isotopes for nuclear-charge-radii determination

Abstract

Changes in the mean square nuclear charge radii along the lithium isotopic chain were determined using a combination of precise isotope shift measurements and theoretical atomic structure calculations. Nuclear charge radii of light elements are of high interest due to the appearance of the nuclear halo phenomenon in this region of the nuclear chart. During the past years we have developed a laser spectroscopic approach to determine the charge radii of lithium isotopes which combines high sensitivity, speed, and accuracy to measure the extremely small field shift of an 8-ms-lifetime isotope with production rates on the order of only 10 000 atoms/s. The method was applied to all bound isotopes of lithium including the two-neutron halo isotope 11 Li at the on-line isotope separators at GSI, Darmstadt, Germany, and at TRIUMF, Vancouver, Canada. We describe the laser spectroscopic method in detail, present updated and improved values from theory and experiment, and discuss the results.

Dynamic Stark shift of the7Li(2s→3s) transition

“…The first stage in the Li(2s → 3s) transition frequency experiment [1,3] involves the two-photon excitation to the 3s level. The laser photon with λ = 735.1 nm, emitted by a Ti:sapphire laser, has an energy of 0.061 98 a.u..…”

“…High precision transition frequencies for the Li(2s → 3s) transition have been measured for the 6 Li and 7 Li atoms [1][2][3]. The experimental procedure involves two-photon excitation from the 2s ground state to the 3s level, a decay to the 2p level, a single-photon excitation to the 3d level, and finally photoionization driven by the laser fields used to excite the atom.…”

Tensorial form and matrix elements of the relativistic nuclear recoil operator

“…More interesting is the comparison with the recent SMS values of Kozhedub et al [26] investigating the relativistic recoil corrections and using the same NMS and SMS partition according to (5) and (51), the FS contribution should be properly subtracted from the observed transition frequency before trying to extract the mass contribution. But this is usually the other way round that makes the theoretical calculation of mass shifts interesting: for a few-electron atomic systems like lithium indeed, the difference between the mass contribution calculated by elaborate ab initio calculations and the observed transition IS allows us to extract the change in the mean square charge radius of the nuclear charge distributions for all isotopes, as illustrated by the very recent and complete work of Nörtershäuser et al [28]. Another good reason is that once the FS 'eliminated', a clean separation of the NMS and SMS contributions could be criticized, as pointed out by Palmer [4].…”

On the breakdown of the Dirac kinetic energy operator for estimating normal mass shifts

“…The isotope shifts (IS), which consist of the field shift and the mass shift, always play a key role in extracting nuclear properties such as nuclear mean-square charge radii [1][2][3][4][5]. The mass shift, however, generally gives rise to large uncertainties in the IS due to its sensitivity to electron correlation.…”