Precise Measurement of the2P03−2P13Fine-Structure Interval of Helium

The $2{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}-2\mathrm{}{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ fine-structure interval ${\nu }_{01}$ of helium has been measured by the atomicbeam optical-microwave magnetic-resonance method. The result is ${\nu }_{01}=29\mathrm{}616.864\left(36\right)\mathrm{}$ MHz (1.2 ppm), in which the error is 1 standard deviation. This precision is more than a factor of 10 better than that obtained by any other method. Comparison with the present theoretical value of ${\nu }_{01}$ is given, and the possibility of obtaining a value for the fine-structure constant $\alpha$ accurate to better than 1 ppm from our experimental result is emphasized.