Many-body calculations of atomic isotope shifts

The specific mass shift contribution to the isotopic shift in ionisation energies or selected transition frequencies has been computed for a series of atoms ranging from lithium to potassium. Many-body perturbation theory within the algebraic approximation was used to compute separate expectation values of the specific mass shift Hamiltonian for upper and lower states, and the computations are complete to third order. The nuclear field shift was also considered in some cases, and computed to third order. Good agreements with experiments were obtained in most of the cases investigated. The computed residual isotopic shift (specific+field shift) between 25 Na and 23 Na for the 2 P- 2 S transition is 386.5 MHz, compared with an experimental shift of 375.4(1.3) MHz. Similarly the computed residual shift in the potassium ionisation energy ( 41 K- 39 K) is found to be -59.8 MHz, in complete agreement with the experimental value of -61(2) MHz.