Electron temperature and density dependence of E1 and E2 lines in the spectra of cobaltlike to potassiumlike ions

We report on a model for the intensity of ''forbidden'' 3dk-3dk-14s electric quadrupole (E2) transitions in ions with ground states of the form 3p63dk (k=1 through 9) across a broad range of temperatures and densities. (Molybdenum, Z=42, is a representative element because of its role in magnetically confined fusion experiments and the availability of experimental data.) We make an identification of a 3d8-3d74s E2 line in MoXVII. In ions where strong E2 lines are not seen (ions with ground states 3dk, 1≤k≤7), the suppression of the lines is explained by dipole decays to low-lying levels other than the ground state that become allowed through configuration interaction between 3dk-14s states and excited states that have electric dipole (E1) decays to the 3p53dk+1 levels (and in like manner mixing between the 3p53dk+1 configuration and other configurations with E1 decays to the levels of 3dk-14s). For k=8 and 9, we find that radiative cascades from high-lying levels play an important role in populating upper states of the E2 transitions. The role of direct, collisional ionization from valence and inner subshells of adjacent charge states in populating the upper states of 3d-4p E1 and 3d-4s E2 decays in MoXVI and MoXVII is found to be negligible. General agreement is found between observations of the E2 to E1 brightness ratio of MoXVI made in a tokamak plasma and the present model. Sensitivity to changes in electron density in the ratio of E2 to E1 decays can be used as a diagnostic for fusion plasmas. copyright 1996 The American Physical Society