Detection of Rydberg states in atomic uranium using time-resolved stepwise laser photoionization

We report the first observation of perturbed and unperturbed Rydberg progressions of atomic uranium. Highlying levels within 1000 cm^-1 of the ionization limit were accessed by time-resolved stepwise excitation using dye laser pulses tuned to resonant transitions. Atoms excited to these states were then photoionized by intense infrared radiation from a pulsed CO₂ laser. The resultant photoion production was monitored. By delaying the infrared ionizing pulse, and thus discriminating against the shorter-lived valence states, we preferentially detected Rydberg levels with principal quantum numbers n exceeding 60. Series convergence yields a value of the ionization limit of 6.1941 ±{} 0.0005 eV, in fair agreement with the value of 6.1912 ±{} 0.0025 eV obtained in photoionization studies. Ab initio calculations indicate that the unperturbed series belong to highly excited 5f³7s²np and nf configurations.