Resonant laser ionization of neptunium: investigation on excitation schemes and the first ionization potential

The atomic structure of neptunium (Np) was investigated by two-step resonance ionization spectroscopy. The study involved exploring ground-state transitions as well as following transitions to high-lying states just below the ionization potential (IP) or auto-ionizing states above the IP. That resulted in the identification of two-step ionization schemes, suitable for trace analysis and nuclear structure investigations. The lifetimes of two excited states located at 25,342.48 cm^-1 and 25,277.64 cm^-1 were determined as 230(12) ns and 173(9) ns, respectively. Because of the absence of Rydberg series in wide-ranging spectra recorded, the first IP was determined through the field ionization of high-lying, weakly-bound states using a well-controlled static electric field. By applying the saddle-point model, an IP value of 50,535.54(15) cm ^-1 [6.265608(19) eV] was derived. This value agrees with the current literature value of 50,535(2) cm ^-1 , while providing a more than ten times higher precision.