Electron collisional excitation in F-like selenium

Electron collisional excitation of the low-lying 1s²2s²2p⁵ and 1s²2s2p⁶ states of F-like selenium to the singly excited M-shell states is studied using a relativistic distorted-wave model and multiconfigurational relativistic Hartree-Fock bound states. Results are presented for all 2-3 transitions from the low-lying 2s²2p⁵ ²P3/2 and ²P1/2 levels, and also the 2s2p⁶ ²S1/2 level. We find a number of strong dipole-allowed 2p-3d cross sections with peak values near threshold in excess of 10^-20 cm², and derive Gaunt factors which are in good agreement with values used in the literature (0.15--0.20) for most strong transitions.Very strong monopole 2p-3p excitation cross sections have been important in soft-x-ray laser theory, and are found to be as strong as the largest dipole-allowed cross sections for F-like selenium. Theoretical output powers for the strong lines of the 2p-3s, 2p-3d, and 2s-3p transition arrays are computed and presented for plasma conditions of Nₑ=3×{}10²⁰ cm^-3 and Tₑ=1.0 keV. These results are compared in detail for proportionality against gf values for each array separately, as a test of how well line intensities might be judged from gf values in the absence of detailed theoretical intensity results. We find that for the 2p-3s and 2p-3d arrays, the intensities are in fair agreement with gf values within the array, while the agreement is much poorer in the case of the 2s-3p array. The two weaker arrays 2p-3s and 2s-3p are found to radiate more per unit gf than the 2p-3d transition array, in agreement with earlier observations in the Ne-like sequence. Theoretical line positions are tabulated for all strong 2-3 lines, and found to be in good agreement with experimental results for most strong transitions. Gains on the 3-3 transitions in between 2 and 4 cm^-1 are predicted for four lines under the plasma conditions quoted. Such conditions are similar to those of the recent extreme-uv laser experiments at the Lawrence Livermore National Laboratory, yet no F-like 3-3 amplification has yet been observed for F-like transitions. The discrepancy is currently a mystery.