Investigation of fine-structure quantum beats in sodium Rydberg atoms by field ionization

Pulsed dye lasers are used to excite sodium atoms into a coherent superposition of high Rydberg $\mathrm{nd} ({}_{}{}^2{}_{}{}^{}\mathrm{D}_{\frac{3}{2}}^{}{}_{}{}^{},{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{})$ states. Analysis of the field ionization behavior of these atoms reveals the time evolution of the superposition state. Rapid application of a weak electric field projects different components of the superposition state onto states having identifiably different field ionization behavior. Analysis of the signal from subsequent field ionization as a function of the time of weak-field application reveals fine-structure quantum beats. These beats result from the time development of the original superposition state. Measurements of the fine-structure quantum beat frequency are presented for the principal quantum numbers $n=32, 34, 35, 36, 38, \mathrm{and} 40$.