We report the observation of fine-structure and Zeeman quantum beats in highly excited sodium
D states prepared, for the first time in a quantum-beat experiment, by stepwise excitation using two synchronously pulsed dye lasers. We have used this new technique to measure the previously unknown fine-structure intervals of levels
9D and
10D of sodium, the values being, respectively, 124.5\ifmmode\pm\else\textpm\fi{}1.5 and 91.5\ifmmode\pm\else\textpm\fi{}1 MHz.
Pulsed dye lasers are used to excite sodium atoms into a coherent superposition of high Rydberg
nd(2D23,2D25) 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,and40.
Millimeter resonances corresponding to single- and double-photon transitions have been observed between Rydberg levels of sodium with principal quantum number
n ranging from 23 to 41. The levels are prepared by laser excitation; transitions are induced by microwaves produced by a backwave oscillator working around 300 GHz; detection is performed through the selective field-ionization technique. Experimental values for quantum defects, fine-structure intervals, and polarizabilities are reported.