The $2{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$-to- $2{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ interval in atomic helium is measured using a thermal beam of metastable helium atoms excited to the $2{}_{}{}^3{}_{}{}^{}P$ state using a $1.08-\mu \mathrm{m}$ diode laser. The $2{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$-to- $2{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ transition is driven by 2.29-GHz microwaves in a coaxial transmission line. Our result of $2291174.0\pm 1.4\mathrm{kHz}$ is the most precise measurement of helium $2{}_{}{}^3{}_{}{}^{}P$ fine structure. This measurement plays a key role in obtaining a new value for the fine-structure constant.

• Received 15 December 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.3274