Shifts due to quantum-mechanical interference from distant neighboring resonances for saturated fluorescence spectroscopy of the 23S to 23P intervals of helium

Quantum-mechanical interference with distant neighboring resonances is found to cause shifts for precision saturated fluorescence spectroscopy of the atomic helium 2(3)S-to-2(3)P transitions. The shifts are significant (larger than the experimental uncertainties for measurements of the intervals) despite the fact that the neighboring resonances are separated from the measured resonances by 1400 and 20 000 natural widths. The shifts depend strongly on experimental parameters such as the angular position of the fluorescence detector, the intensity and size of laser beams, and the properties of the atomic beam. These shifts must be considered for the ongoing program of determining the fine-structure constant from the helium 2(3)P fine structure.