Fine Structure of the $1s3p{}^{3}P_J$ Level in Atomic ${}^4\mathrm{He}$: Theory and Experiment

The fine structure intervals in helium have been the focus of many theoretical and experimental studies in recent years with most of them concentrating on the $1s2p{}^{3}P_J$ levels. Here, we report on a theoretical calculation and an experimental determination of the $1s2p{}^{3}P_J$ fine structure intervals. The values from the theoretical calculation are 8113.730(6) and 658.801(6) MHz for the ${\nu }_{01}$ and ${\nu }_{12}$ intervals, respectively. The laser spectroscopic measurement reported here yields 8113.714(28) and 658.810(18) MHz for these intervals and is in excellent agreement with the theoretical calculation. Both, however, disagree significantly with the previous most precise experimental results.

Figure 1

Schematic of the experimental setup used for the detection of laser induced fluorescence from a beam of metastable helium atoms.

Figure 2

Typical resonance profile for the $1s2s{}^{3}S_1\to 1s3p{}^{3}P_2$ transition taken at a laser power of $50\mu \mathrm{W}$. The dashed line is a least-square fit of a Voigt profile to the data. The fit uncertainty on the line center is 23 kHz. The respective residuals are shown in the lower plot.

Figure 3

Experimental fine structure splittings for (a) ${\nu }_{01}(J=0\to 1)$ and (b) ${\nu }_{12}(J=1\to 2)$ as a function of laser power; the dashed lines are linear fits to the data to extract the zero-power values. A laser power of $350\mu \mathrm{W}$ corresponds to an intensity of $\sim 0.45\mathrm{mW}/{\mathrm{cm}}^2$ or $\sim 13\%$ of the saturation intensity of $3.4\mathrm{mW}/{\mathrm{cm}}^2$.

Figure 4

Comparison of results for the three fine structure intervals of the $1s3p{}^{3}P_J$ levels from Wieder et al. [13], Kramer et al. [14], and Yang et al. [6], as well as the experimental and theoretical values from this work. The theoretical values are also indicated by the dashed lines.