Fine-structure intervals and polarizabilities of highly excitedpanddstates of sodium

Abstract

Selective field ionization of the $\left|m_j\right|$ fine-structure levels of highly excited $p$ and $d$ states of Na has been used to measure the fine-structure (fs) intervals and tensor polarizabilities of the $p$ states from $n=16\mathrm{} \mathrm{to} 19$ and $d$ states from $n=15\mathrm{} \mathrm{to} 17$ by rf spectroscopy. Using a two-step photo-excitation scheme we selectively excite one fs state of a $p$ or $d$ level and set the ionizing field so that it will field ionize the other fs state but not the one pumped by the lasers. Consequently, as the rf frequency is swept through the fs resonance, a substantial increase in the ionization current is observed. Our measurements show that the $p$ fs intervals follow an (${n^{*}}^{-3\mathrm{}}$) rather than an $n^{-3\mathrm{}}$ scaling law ($n^{*}$ is the effective principal quantum number). The polarizabilities of both $p$ and $d$ states are in good agreement with calculations using a Coulomb approximation. The measurements of the $d$-state polarizabilities now cover a wide enough range of $n$ to confirm the theoretical prediction of an $n^7$ scaling law.