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'' 3 d(k)-3 d(k-1)4s electric quadrupole (E2) transitions in ions with ground states of the form 3p(6)3d(k) (k = 1 through 9) across a broad range of temperatures and densities. (Molybdenum, Z = 42, has been chosen as a representative element because of its role in magnetically confined fusion experiments and the availability of experimental data.) We make an identification of a 3d(8)-3d(7)4s E2 line in Mo XVII. In ions where strong E2 lines are not seen (ions with ground states 3d(k), 1 less than or equal to k less than or equal to 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 3d(k-1)4s states and excited states that have electric dipole (E1) decays to the 3p(5)3d(k+1) levels (and in like manner mixing between the 3p(5)3d(k+1) configuration and other configurations with E1 decays to the levels of 3d(k-1)4s). 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 Mo XVI and Mo XVII is found to be negligible. General agreement is found between observations of the E2 to E1 brightness ratio of Mo XVI made in a tokamak plasma and the predictions of 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.