Supercomplex spectra and continuum emission from rare-earth ions: Sm, a case study

Abstract

Spectra from laser-produced plasmas of elements within the range 62 ≤ Z ≤ 74 (Z is the atomic number) are known to contain extensive regions of line-free continua throughout the VUV and XUV regions. These continua arise primarily from recombination while strong line emission is inhibited by the complexity of the electronic configurations involved which, as a result of 4f collapse, for the most part contain an open 4f subshell. The energies and compositions of the lowest configurations of rare-earth ions along the I-, Xe- and Cs-isoelectronic sequences are investigated and it is concluded that they consist of fixed parity sets of highly mixed or 'compound' states built from configurations containing variable numbers of 5s, 5p and 4f electrons. These are described within the statistical unresolved transition array model. The strongest transitions which occur in these sequences for ionized samarium are explored using configuration-averaged Hartree-Fock calculations, while detailed line and level statistics are extracted for the lowest 4f → 5d and 5p → 5d transitions for Sm IX. The calculations predict a redistribution of single-configuration oscillator strengths from the stronger lines to the weaker ones in a full configuration-interaction (CI) basis which essentially smooths the spectra. It is concluded that both the level density and CI effects are such as to produce level distributions equivalent to nuclear compound states and combine to produce emission so complex that it is essentially 'band-band' in nature (supercomplex spectra) leading to a spectrum which is a line-free continuum.