Narrow (Δλ<5 Å), intense quasicontinuum bands, appearing in emission spectra of highly ionized rare-earth elements between 70 and 100 Å, 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 4$p^6$4$d^{N\mathrm{-}1}$4f and 4$p^5$4$d^{N+1\mathrm{}}$ 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.

• Received 21 January 1987

DOI:https://doi.org/10.1103/PhysRevA.35.5051