Interpretation of the quasicontinuum band emitted by highly ionized rare-earth elements in the<i>70–100</i>-<i>Å</i>range

Abstract

Narrow (\ensuremath{\Delta}\ensuremath{\lambda}5 \AA{}), intense quasicontinuum bands, appearing in emission spectra of highly ionized rare-earth elements between 70 and 100 \AA{}, 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{\ensuremath{-}}1}$4f and 4${p}^{5}$4${d}^{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.