Helium 2³p fine structure measurement in a discharge cell

A precise measurement of helium 2³P fine structure was carried out in a discharge cell using Doppler-free laser spectroscopy. It is the only known experiment to directly measure all three fine structure intervals at a 1 kHz level of accuracy. The 2³P1 - 2³P2 interval value agrees with other experiments but disagrees with theoretical predictions of two-electron QED. When this disagreement is resolved, the 2³Po - 2³P1 interval measurement reported here will allow a determination of the fine structure constant to 14 parts in l0₉, surpassing the precision of the well known QED-independent quantum Hall effect and Josephson effect determinations. The discharge cell is shown to be advantageous in the study and correction of systematic frequency shifts related to light pressure, and the use of the cell ensures that the possible systematic errors are substantially different from those reported in other experiments.