Ultraviolet-infrared double-resonance laser spectroscopy ofnd(n=12−17) Rydberg states inHe3

The hyperfine structure of the singlet and triplet $\mathrm{nd}$ Rydberg states ($n=12-17\mathrm{}$) in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ have been studied by Doppler-free ultraviolet-infrared double-resonance laser spectroscopy. Using the frequency-doubled output of a cw dye ring laser we have populated the $1s5p{}_{}{}^3{}_{}{}^{}P$ state of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ by excitation from the metastable $1s2s{}_{}{}^3{}_{}{}^{}S$ state at 294.5 nm. In a second step, an infrared single-mode cw color center laser is used to excite $d$ Rydberg states. The experimental results for the $n{}_{}{}^{1,3}{}_{}{}^{}D$ hyperfine splittings ($n=12-17\mathrm{}$) are in excellent agreement with semiempirical calculations using hydrogenlike wave functions. Anomalous isotopic shifts in the electrostatic energy intervals of 5 to 20 MHz were found for the states $n=12-17\mathrm{}$. These shifts are smaller than the shifts of about 35 MHz found for the states $n=5-8\mathrm{}$ with the help of anticrossing spectroscopy. No satisfactory explanation of these shifts has yet been found.