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 10³ atoms/s in a hot cavity. We measured the wave numbers of the 7p²{P}3/2→{nd}²$D (n=22--33) and 7${p}²P3/2{→}ns²S (n=23, 25--27,29--31) transitions and found the binding energy of the 7p²{P}3/2$ state to be T=-18 924.8(3) ${cm}^{{{-}}1}$, which made it possible to establish accurately the ionization potential of Fr.