Calculation of radiative corrections to hyperfine splittings in the neutral alkali metals

The radiative correction to hyperfine splitting in hydrogen is dominated by the Schwinger term, α/2πE_F, where E_F is the lowest-order hyperfine splitting. Binding corrections to this term, which enter as powers and logarithms of Zα, can be expected to be increasingly important in atoms with higher nuclear charge Z. Methods that include all orders of Zα, developed first to study highly charged ions, are adapted to the study of the neutral alkali metals, lithium through francium. It is shown that the use of the Schwinger term alone to account for radiative corrections to hyperfine splittings becomes qualitatively incorrect for the heavier alkali metals.