Abstract

The third spectrum of dysprosium has been observed in a sliding-spark discharge with a 10.7-m normal-incidence spectrograph in the wavelength region between 2007 and 5020 Å. Of the 2500 lines that were attributed to Dy iii, 544 are classified. With these data 106 energy levels were established, including five of the ${ }^{5}I$ ground term of the $4f^{10}$ configuration, 49 of $4f^{9}5d,$ 15 of $4f^{9}6s,$ and 37 of $4f^{9}6p.$ The levels have been interpreted theoretically and have been assigned percentage compositions in the most appropriate coupling schemes. Using semiempirical calculations with least-squares-fitted radial parameters, we have obtained average deviations between experimental and calculated levels of $\pm 49,$ $\pm 29,$ and $\pm 16 {\mathrm{cm}}^{-1}$ for the $4f^{9}5d,$ the $4f^{9}6p,$ and the $4f^{9}6s$ configurations, respectively. The overlapping configurations $4f^{9}5d$ and $4f^{9}6s$ show slight evidence of configuration interaction. This analysis leaves Pm iii as the last spectrum of a doubly ionized rare earth to remain uninterpreted. With these data, a value of 184 970 $\pm$ 800 ${\mathrm{cm}}^{-1}$ has been derived for the ionization energy.

© 1997 Optical Society of America

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