Spectrum and energy levels of five-times-ionized niobium (Nb VI)

The spectrum of the kryptonlike ion Nb VI was observed from 325 to 2700 with sliding-spark discharges on 10.7-m normal-incidence and grazing-incidence spectrographs. Experimental energies were determined for all levels of the 4[ital s][sup 2]4[ital p][sup 6], 4[ital s][sup 2]4[ital p][sup 5]4[ital d], 4[ital f], 5[ital s], 5[ital p], 5[ital d], 5[ital g], 6[ital s], and 4[ital s]4[ital p][sup 6]4[ital d] configurations as well as for some levels of the 4[ital s][sup 2]4[ital p][sup 5]6[ital g] and 6[ital h] configurations. A total of 303 lines are now classified as transitions between 99 observed levels. Large hyperfine splittings were found for several levels of the 4[ital p][sup 5]5[ital s] and 5[ital p] configurations. The observed configurations were theoretically interpreted by means of Hartree--Fock calculations and least-squares fits of the energy parameters to the observed levels. An improved value of the ionization energy was determined from the 4[ital p][sup 5]5[ital g], 6[ital g], and 6[ital h] configurations. The adopted value of the limit is 823 240 [plus minus] 50 cm[sup [minus] 1] (102.069 [plus minus] 0.006 eV).