Progress at NIST in measuring the D-lines of Li isotopes using an optical frequency synthesizer

Precise spectroscopic experiments with light atoms can provide information about nuclear properties that are very difficult to obtain in electron scattering experiments. For example, relative nuclear radii of low-Z isotopes can be determined accurately from isotope shifts. Theory has attained sufficient accuracy to study exotic, short-lived halo nuclei by interpreting precise spectroscopic measurements. However, serious inconsistencies remain in the measured isotope shifts for the D1 and D2 lines of the stable isotopes (Li-6 and Li-7). The latest experiments, within the last decade, are in strong disagreement with each other and with theory. We report on the progress of a new experiment at the National Institute of Standards and Technology (NIST) to measure these lithium D lines using an optical frequency comb. A preliminary result for the splitting isotope shift (SIS) is presented.