Determination of the atomic-hydrogen fine structure by level crossing in the 2P states of hydrogen, a measurement of the fine-structure constant

The Sommerfeld fine-structure constant has been determined from a direct measurement of the fine-structure separation in the 2P states of atomic hydrogen using the level-crossing technique. From the changes in resonance fluorescence caused by level crossing at 3484 G, we obtain a value for the fine-structure separation E(P322)-E(P122)=10 969.13(0.10) MHz and -1=137.0354(6). The quoted uncertainty represents our estimate of 70% confidence from the analysis of 84 separate runs.