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

Spectra from laser-produced plasmas of elements within the range 62 less th an or equal to Z less than or equal to 74 (Z is the atomic number) are know n to contain extensive regions of line-free continua throughout the VUV and XUV regions. These continua arise primarily from reeombination while stron g line emission is inhibited by the complexity of the electronic configurat ions involved which, as a result of 4f collapse, for the most part contain an open 4f subshell. The energies and compositions of the lowest configurat ions of rare-earth ions along the I-, Xe- and Cs-isoelectronic sequences ar e investigated and it is concluded that they consist of fixed parity sets o f highly mixed or 'compound' states built from configurations containing va riable numbers of 5s, 5p and 4f electrons. These are described within the s tatistical unresolved transition array model. The strongest transitions whi ch occur in these sequences for ionized samarium are explored using configu ration-averaged Hartree-Fock calculations, while detailed line and level st atistics are extracted for the lowest 4f --> 5d and 5p --> 5d transitions f or 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 combi ne to produce emission so complex that it is essentially 'band-band' in nat ure (supercomplex spectra) leading to a spectrum which is a line-free conti nuum.