Investigation on the DC Stark shifts of strontium autoionization states for isotope-selective resonance ionization

Highlights: • We measured for the first time the DC Stark shifts of ten Rydberg levels (n${}_{eff}$ 36–39, 50–55) of ⁸⁸Sr converging to the 4d${}^2$ D${}_{3/2}$ limit. • The polarizability $\alpha$ of each level was evaluated to be $\left|\alpha \right|{10}^{-30}-{10}^{-28}\left[C^2{m^{2}J}^{-1}\right]$ depending on the effective principal quantum number n_eff. • The residual electric field in the ionization region should be of the order of 0.1 V/cm or less for trace analysis of radioisotope ${}^{90}$Sr using three-step resonance excitation. • For some Rydberg levels, another nearby level was also observed with the applied electric field, implying that the DC Stark shift allows observation of forbidden transitions. Three-step resonance excitation to autoionization states is a promising isotope-selective ionization technique for atomic strontium (Sr). We measured for the first time the DC Stark shifts of ten Rydberg levels ($n_{\text{eff}}\sim 36-39,$ $50-55$) of ${}^{88}$Sr converging to the 4d ${}^2$D${}_{3/2}$ limit using the 5s${}^2{}^1$S${}_0$ $\to$ 5s5p ${}^1$P${}_1^{\circ }$ $\to$ 5p${}^2{}^1$D${}_2$ $\to$ (4d ${}^2$D${}_{3/2}$ Rydberg series) transition pathway. The polarizability $\alpha$ of each level was evaluated to be $\left|\alpha \right|\sim {10}^{-30}-{10}^{-28}$ [C${}^2$$·$m${}^2$$·$J${}^{-1}$] depending on the effective principal quantum number $n_{\text{eff}},$ which indicates that the external electric field $E$ in the ionization region should be of the order of 0.1 V/cm or less to suppress its effect on the observed spectra. Our experimental data on the DC Stark shift and polarizability of the Rydberg levels are useful for future isotope-selective detection of the radioisotope ${}^{90}$Sr using three-step resonance excitation.