Precision microwave measurement of the23P1–23P0interval in atomic helium

We have measured the $2^3{P}_1–2^3{P}_0$ interval in atomic helium to be 29 616.966(13) MHz (an accuracy of 400 ppb). The microwave transition is driven using a laser-excited thermal beam of metastable helium atoms. The result of the measurement resolves the discrepancy between a previous, less-precise microwave measurement and recent laser measurements of this interval. Comparison of measured and theoretical predictions for this interval allows for a determination of the fine-structure constant.