Ground-state configurations of ionic species I through XVI forZ=57−74and the interpretation of4d−4femission resonances in laser-produced plasmas

Abstract

Spectra of the elements barium ($Z=56$) through Nd ($Z=60$) observed in laser-produced plasmas from pure-metal targets in the wavelength region 70- 130 \AA{} are presented. This completes our survey of the spectra of elements from cesium to hafnium in two regimes of optical thickness. The two sets of spectra show significant differences. In order to interpret the whole range of results, we need to know the ground-state configurations of ions in stages up to XVI for $Z=55\ensuremath{-}71$. From the limited-term analyses available, a table of plausible ground configurations is proposed. In the preparation of the table, the effect of increasing ionization, for a fixed $Z$, on wave-function contraction is studied, and it is inferred that the $4f$ electron becomes more tightly bound than $5p$ between the sixth and seventh spectra of the elements from cerium to lutetium; similarly the $4f$ electron attains a higher binding energy than does the $5s$ between the fourteenth and fifteenth spectra. Consequently, for a considerable range of ion stages, particularly in the rare earths, the ground configurations have a partially filled $4f$ shell. Furthermore, in the same ion stages, the $4f$, $5p$, and $5s$ energies stay relatively close together so that configurations of the type $5{p}^{l}4{f}^{n\ensuremath{-}l}$ lie very near the nominal ground configuration. A corresponding manifold of overlapping excited configurations of the type $4{d}^{9}4{f}^{n+1}$, $4{d}^{9}5{p}^{l}4{f}^{n\ensuremath{-}l+1}$ is also expected and the observed resonances arise from all of the $4d\ensuremath{-}4f$ transitions permitted between the upper and lower manifolds of configurations. The numbers of transitions are calculated and in the majority of cases are very great. Because the principal quantum number does not change, the transitions will tend to overlap for different ion stages of any one element. The widths and varying complexity of the resonances can be explained in a satisfactory, if qualitative, way by means of available theory. The very considerable difference between the compound and metal spectra can be explained in terms of the effect of number density of the active species on both emission and absorption processes in the two types of plasma.