Photoionization and Rydberg spectra of ten lanthanides have been studied using stepwise laser excitation and ionization methods. These spectra Were obtained from several different laser populated excited states in each case. Ac~urate ionization limits were derived from observed photoionization thresholds. Except for praseodymlulll, the observation of one or more long Rydberg progressions allowed more accurate limits to be determined.
We report the first observation of perturbed and unperturbed Rydberg progressions of atomic uranium. Highlying levels within 1000 cm ' 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 5f'7s'np and nf configurations.
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