Shifts due to neighboring resonances for microwave measurements of the 2 3P fine structure of helium

Quantum-mechanical interference with far-off-resonance neighboring states is found to cause systematic shifts for the measurements of the 2 P-3 fine-structure intervals. The shifts depend on the type of experiment used to measure the intervals. Here the shifts are calculated for measurements using a single microwave pulse and for measurements using the Ramsey method of separated oscillatory fields. The shifts are small, but are large enough to affect the continuing program of determining the fine-structure constant from a comparison between accurate experimental measurements and theoretical calculations of the interval energies. The separated-oscillatory-field shifts are found to be much smaller than the single-pulse shifts.