Saturated fluorescence spectroscopy measurement apparatus based on metastable Li⁺ beam with low energy

The apparatus for fluorescence spectroscopy measurement is developed to determine the fine structure (FS)/hyperfine structure (HFS) splittings of 1s2p ³P_J(J=0,1,2) states of Li⁺. The instrument is composed of a low energy Li⁺ ion source and a saturated fluorescence spectroscopic probing system. A low energy Li⁺ ion source, containing 1.8(7)% of 1s2s ³S₁ metastable ions with an energy of ∼500 eV, is obtained by an electron bombardment process. The ion current can stay more than 250 h with the variation of ∼0.3%, and the divergence of ion beam is ∼0.5 mrad. The symmetric profile of Lamb dip signals of 1s2s ³S₁--1s2p ³P_J transitions with linewidths of ∼50 MHz are obtained after subtracting Doppler background from the saturated fluorescence signals. A back-and-forth scan method is adopted to determine the FS/HFS splittings of 1s2p ³P_J states. Under these conditions, as a preliminary test, several splittings of ⁷Li⁺ are measured. The statistical uncertainties of the FS/HFS splittings are estimated to be less than 50 kHz, and the results are one order of magnitude better than previous results. The apparatus is feasible to precisely determine the splittings of energy levels of alkali and alkaline earth ions.