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.

Coherent and stochastic contributions of compound resonances in atomic processes: Electron recombination, photoionization, and scattering

“…Electron capture in chaotic multielectronic resonances in open-shell ions results in recombination rates 10 2 -10 3 times greater than the singleparticle radiative recombination rate [5,[14][15][16], as seen in many experiments [17][18][19][20][21][22]. Similar states feature in photoionization and photoemission in many ions, producing a complex interplay of broad and narrow resonances [23][24][25][26][27][28][29]. IVR is an essential step in most chemical reactions.…”

Ground-state configurations and unresolved transition arrays in extreme ultraviolet spectra of lanthanide ions

“…In this were calculated. The ground configuration was taken as the one with the lowest calculated average energy [33,34] and these results are illustrated comprehensively in Fig. 1 and Table II. As noted by Carroll and O'Sullivan [22], the behaviour of the 4f electron is crucial for an understanding of the ground-state configurations of the lanthanides in their various stages of ionization.…”

Signatures of quantum chaos in rare-earth elements: II. Characterization of the energy eigenvalues and dipole moments of CeIand PrI

“…According to arguments given in [12], one would expect the level density to be closer to a Gaussian shape than to a semi-circle shape. Gaussian-like level densities can be seen in [13] and possibly in odd Pr-see figure 30 of [1]. The transition from Gaussian to semi-circle level density occurs [12,14,15] when many-body forces are introduced, lifting the selection rules for interactions between configurations of different angular momentum.…”

“…In [13] the effect of CI and non-CI on the nearest neighbour spacing (NNS) of Sm IX was highlighted. It was found that CI gave rise to a Wigner NNS distribution, whereas the non-CI gave rise to a Poisson distribution.…”