Interpretation of the quasicontinuum band emitted by highly ionized rare-earth elements in the 70--100-A range

Narrow (Δλ<5 A), intense quasicontinuum bands, appearing in emission spectra of highly ionized rare-earth elements between 70 and 100 A, previously observed in tokamaks and laser-produced plasmas, have been obtained from a low-inductance vacuum spark. The bands shift toward shorter wavelengths with increasing atomic number Z. Using the unresolved transition array model, these bands are identified as primarily 4d-4f transitions in Rh I to Rb I--like ions, although the widths come out too large and the mean wavelengths are much too dependent on ionization stages. Detailed ab initio computations show that the interactions between the 4p64d/sup N-1/4f and 4p54d/sup N+1/ configurations are responsible for the narrowing and the superposition of the transition arrays for the different ionization stages of a given element, in agreement with experimental data