Laser-spectroscopy measurement of the fine-structure splitting $2{}^3{P}_1\text{–}2{}^3{P}_2$ of ${}^4\mathrm{He}$

Laser spectroscopy has been performed on a beam of neutral ${}^4\mathrm{He}$ 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 $2{}^3{P}_1\leftarrow 2{}^3{S}_1$ transition at the wavelength of 1083 nm, followed by laser spectroscopy on the $2{}^3{P}_{1,2}\leftarrow 2{}^3{S}_1$ transitions. The $2{}^3{P}_1-2{}^3{P}_2$ fine-structure splitting is determined to be $2291177.69\pm 0.36\mathrm{kHz}$. 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.

Figure 1

Comparison of the ${\nu }_{12}$ values from experimental and theoretical studies.

Figure 2

Configurations of the experimental setup (a) and the schemes for optical pumping (b) and spectral excitation (c).

Figure 3

Spectrum of the $2{}^3{P}_1\leftarrow 2{}^3{S}_1$ and $2{}^3{P}_2\leftarrow 2{}^3{S}_1$ transitions obtained from a single scan. The scattered points are experimental data and the continuous curve is the simulated spectrum. The deviations between experimental and simulated spectra are shown in the lower panels.

Figure 4

Dependence of the measured frequency interval on the probe laser power.

Figure 5

The $2{}^3{P}_2\text{–}2{}^3{P}_1$ splitting obtained at different magnetic fields. The dots are the measured values and the solid curve is for the calculated second-order Zeeman shifts. The differences are given in the lower panel. The squares and triangles represent the transitions measured using ${\sigma }^{+}(m=+1)$ and ${\sigma }^{-}(m=-1)$ circularly polarized laser beams, respectively.

Figure 6

Investigation of systematic deviations due to (a) Doppler shift, (b) misalignment of the laser polarization, and (c) rf power (normalized) applied on the EOM to produce sidebands on the probe laser.

Figure 7

Investigation on the potential systematic deviations.