Higher-Order Nuclear-Size Corrections in Atomic Hydrogen

Nuclear-size corrections of order $(Z \alpha)^5$ and $(Z \alpha)^6$ to the S-state levels of hydrogenic atoms are considered. These nuclear-elastic contributions are somewhat smaller than the polarizability (nuclear-inelastic) corrections for deuterium, but are of comparable or larger size for the hydrogen case. For deuterium the (attractive) nonrelativistic $(Z \alpha)^5$ correction to the 2S-1S transition is 0.49 kHz, while the (repulsive) relativistic $(Z \alpha)^6$ contribution is -3.40 kHz. For the proton the corresponding corrections are 0.03 kHz and -0.61 kHz, respectively. The $(Z \alpha)^5$ contribution largely cancels the Coulomb-retardation part of the nuclear-polarization correction.