Laser resonance photoionization spectroscopy of Rydberg levels in Fr

We investigated for the first time the high-lying Rydberg levels in the rare radioactive element francium (Fr). The investigations were conducted by the highly sensitive laser resonance atomic photoionization technique with Fr atoms produced at a rate of about ${10}^3$ atoms/s in a hot cavity. We measured the wave numbers of the 7$p^2$$P_{3/2}$→${\mathrm{nd}}^2$D (n=22–33) and 7$p^2$$P_{3/2}$→${\mathrm{ns}}^2$S (n=23, 25–27,29–31) transitions and found the binding energy of the 7$p^2$$P_{3/2}$ state to be T=-18 924.8(3) ${\mathrm{cm}}^{\mathrm{-}1}$, which made it possible to establish accurately the ionization potential of Fr.