Shifts from a distant neighboring resonance for a four-level atom

In a recent paper [Phys. Rev. A 82, 052519 (2010)], the systematic shifts of a resonance due to quantum-mechanical interference from a distant neighboring resonance were derived. In that paper, the simplest three-level closed system was used to calculate analytic expressions for shifts in the resonant line centers. Here, we extend the analysis to the more relevant four-level system, which consists of two ground states and two excited states and which incorporates the physics of dark states. The shifts are shown to depend on the type of experiment performed and can be much larger than the shifts for the three-level system. The analytic formulas obtained are applied to the 2^0.16em0ex0.16em0ex3S-to-2^0.16em0ex0.16em0ex3P transitions in atomic helium, where significant shifts are found.