The 5f²-Configuration in Doubly Ionized Thorium, Th III

The ¹I₆ state in Th III 5f² is established at 28 350.35 cm^-1 above the ground level 5f6d³H₄, at variance with previous proposals and with Racah's original calculations. Apart from the strong combination 5f^{2 1}I₆ → 5f6d¹H₅ in the infared, it classifies five other observed lines, three of which, though quite weak, have corroborating Zeeman patterns. It is shown, that a new theoretical treatment taking into account the interactions of 5f² with 6d², 6d7s, 7s² and 5f7p not only confirms the new ¹I₆ but also brings the mean error down from 450 to 78.6 cm^-1. The consequences for high odd states with J = 5, 6 are discussed. The precision of the level values of 5f^{2 3}H₅, ³H₆ is investigated by means of new measurements confirming the original values rather than those published more recently. The configuration calculation is also performed using orthogonal operators whereby the error is reduced to 62.2 cm^-1, due to the increased stability in the fitting procedure. A complete table of composition for the 5 configurations is included. In two appendices the definition of the orthogonal operators, respectively the relationship between the conventional and the orthogonal parameters is reviewed.