Doppler-free two-photon spectroscopy was applied to excite hyperfine-induced transitions from the $5s^2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ ground state to the $F=I$ component of the hyperfine multiplet of the $5sns{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ Rydberg series of ${}_{}{}^{87}{}_{}{}^{}\mathrm{Sr}$ ($n=50,\dots ,85\mathrm{}$). The experimental data confirm the interpretation of hyperfine-induced singlet-triplet mixing which is related to the singlet-triplet energy separation.

• Received 31 March 1982

DOI:https://doi.org/10.1103/PhysRevA.26.2990