A 150 ppb Microwave Measurement of the n=2 Triplet P J=1-to-J=2 Fine-Structure Interval of Helium*

The n=2 triplet P J=1-to-J=2 interval in helium has been measured to a precision of 350 Hz using the Ramsey method of separated oscillatory field. This 350 Hz measurement is the most precise to date of the n=2 triplet structure. Comparison between precise measurements of the n=2 triplet P fine structure and theoretical predictions will allow for a precise determination of the fine-structure constant when the current large discrepancy between experiment (PRL \underline {95} 203001; PRL \underline {87} 173002; PRL \underline {84} 4321; Can J Phys \underline {83} 301) and theory (PRL \underline {97} 013002; Can J Phys \underline {80} 1195) is resolved.