Laser-spectroscopy measurement of the fine-structure splitting 2(3)P(1)-2(3)P(2) of He-4

Laser spectroscopy has been performed on a beam of neutral He-4 atoms. By using transverse laser cooling and focusing, we are able to prepare a bright beam of atoms in the metastable state 2(3)S(1) deflected from the original effusive atomic beam. The initial state preparation is completed with optical pumping on the 2(3)P(1) <- 2(3)S(1) transition at the wavelength of 1083 nm, followed by laser spectroscopy on the 2(3)P(1,2) <- 2(3)S(1) transitions. The 2(3)P(1)-2(3)P(2) fine-structure splitting is determined to be 2 291 177.69 +/- 0.36 kHz. The quantum interference effect is included in data extraction. This is the most precise laser spectroscopy measurement of the interval. Our result is in agreement with both the latest QED-based calculation and the most precise measurement conducted with microwave spectroscopy.