Recoil corrections to Rydberg energy levels in helium

Abstract
We study the question of recoil corrections to Rydberg energy levels in helium, making use of a formalism in which the He-ion core is treated as an elementary particle of spin 1/2 and the Rydberg atom is regarded as a relativistic two-body bound state, described by a ‘‘no-pair’’ wave equation. We show that when recoil effects are correctly included, the agreement with experiment for the n=10 levels of He is improved.