Statistics of electric-quadrupole lines in atomic spectra
  In hot plasmas, a temperature of a few tens of eV is sufficient for producing highly stripped ions where multipole transitions become important. At low density, the transitions from tightly bound inner shells lead to electric-quadrupole (E2) lines which are comparable in strength with electric-dipole ones. In this work, we propose analytical formulas for the estimation of the number of E2 lines in a transition array. Such expressions rely on statistical descriptions of electron states and J-levels. A generalized 'J-file' sum rule for E2 lines and the strength-weighted shift and variance of the line energies of a transition array nl^N+1 \rightarrow nl^Nn'l' of inter-configuration E2 lines are also presented. 
Jean-Christophe Pain, Franck Gilleron, Jacques Bauche and Claire   Bauche-Arnoult
physics.atom-ph