Estimation of the specific mass effect in the isotope shifts of energy levels in the optical spectrum of Ba I and Ba II

A graphical method for separating mass and volume effects from purely optical isotope shift data is presented and compared with King's “bunching” method. Recent experimental data on isotope shifts for a wide range of spectral lines in the naturally abundant isotopes of Ba I and Ba II are analysed. Some agreement is found with muonicx-ray data concerning the nuclear size for the isotopes 136, 137 and 138, but there is disagreement (over 20%) for the other isotopes. Thelevel isotope shifts are further parameterised in terms of a linear model, and the specific mass effect is decomposed into sums of one-electron and two-electron shift parameters with respect to the inert-gas like ground state of Ba III.