Hyperfine Structures of Optically Inaccessible Excited Atomic States

Cascade transitions have been used to produce polarized atoms in optically inaccessible excited states. The polarized fluorescent light which is emitted when the atoms decay can be used to measure hyperfine structures, lifetimes, and other atomic parameters of these inaccessible states. Preliminary measurements of the hyperfine interval for the second excited $S$ states of ${\mathrm{K}}^{39}$, ${\mathrm{Rb}}^{85}$, ${\mathrm{Rb}}^{87}$, and ${\mathrm{Cs}}^{133}$ are (in MHz) 46(2), 271(15), 565(40), and 684(50), respectively. To our knowledge, these are the first measurements of excited $S$-state hyperfine intervals in alkali atoms.