The first ionization potential of neptunium has been determined from the photoionization thresholdand from Rydberg series observed by laser spectroscopy techniques. The Rydberg series convergence limits yield the most accurate value of 50 536(4) cm-1 [6.2657(5) eV]. The radiative lifetimes of five levels in the 26 200–29 050-cm-1 range have been measured. New energy levels, 27 odd and 37 even, in the 33 000–37 000-cm-1 range have been determined with approximately ± 0.5-cm-1 precision.
Earl F. Worden and John G. Conway, "Laser spectroscopy of neptunium; first ionization potential, lifetimes and new high-lying energy levels of Np |," J. Opt. Soc. Am. 69, 733-738 (1979)