Fine Structure of thenD2Series in Rubidium near the Ionization Limit

Abstract

Two-photon, Doppler-free spectroscopy with counter-propagating beams from a cw dye laser and a new type of thermionic detector have been used to study the ${}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ levels of Rb for Rydberg states up to $n=85\mathrm{}$, only 16 ${\mathrm{cm}}^{-1\mathrm{}}$ from the ionization limit. It is shown that the fine-structure spacing varies as ${n_{\mathrm{eff}}}^{-3\mathrm{}}$ for $n>\mathrm{}\sim 25$, and as a linear combination of ${n_{\mathrm{eff}}}^{-5\mathrm{}}$ and ${n_{\mathrm{eff}}}^{-3\mathrm{}}$ for $n<\mathrm{}\sim 25$.