Accurate isotope shift measurement in the D1 and D2 line of Ba⁺

The binding energy especially of s-electrons are affected by the finite nuclear size. Isotope shift measurements of electronic transitions provide therefore information about the nuclear charge radius. The sensitivity of the S_1/2→P_1/2 (D1) and the S_1/2 → P_3/2 (D2) transitions in earth alkaline ions to this finite nuclear size effect should be identical apart from a small relativistic contribution, which can be explained with the relativistic admixture of the S_1/2 wave function to the P_1/2 configuration and has been measured for the first time by K. Wendt et al. in Ba⁺. Recent measurements of this effect in Ca⁺ ions have revealed a considerable discrepancy to theoretical predictions. To check whether such a discrepancy is also apparent in the Ba⁺ system, measurements of higher accuracy are required. Therefore, the absolute transition frequencies in several stable barium isotopes have been measured with a collinear/anti-collinear laser spectroscopy approach.