Isotopic shift in atomic fine structure

Nuclear-motion corrections to the atomic fine-structure isotopic shift are computed by use of many-body perturbation theory. The corrections are of three different types: normal mass effects (change of length scale), electron-nucleus relativistic interactions, and higher-order cross terms between the specific-mass operator and spin-dependent interactions. Numerical results are reported and compared with experiments for the ground states of Br, Ar⁺, Cl, Ne⁺, and C. Except for Ne⁺, reasonable agreements are obtained. For the best investigated case of Cl, the theoretical and experimental shifts are, respectively, -25.4 MHz and -24.9(1.0) MHz. In all cases the electron-nucleus relativistic interaction and the cross term between the specific mass-shift Hamiltonian and the spin-orbit interaction are found to yield large and counteracting contributions. The nuclear field-shift contribution to the fine-structure isotopic effect is also investigated for Ar⁺ and found to be of minor importance.