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Electron collisional data are required for population kinetics modeling and spectral predictions of highly ionized ions in high-temperature plasmas. Nickel-like ions are especially interesting for their potentia1 use in soft-x-ray laser schemes pumped by electron collisional excitation and recombination. For highly stripped ions of moderate to high Z, relativistic effects begin to play a role in the atomic-physics calculations. %e have used a relativistic multiconfigurational distorted-wave model for the calculation of electron excitation cross sections and rate coefficients between the 3s'3p 3d' Ni-like Gd ground state and the singly excited states with an X-shell electron.

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Electron collisiona1 excitation of the low-lying 1s 2s 2p' and 1s 2s 2p states of F-like selenium to the singly excited M-shell states is studied using a relativistic distorted-eave model and multiconfigurational relativistic Hartree-Fock bound states. Results are presented for all 2-3 transitions from the low-lying 2s 2p' P3q2 and 'Plq2 levels, and also the 2s2@ Sl&2 level. %'e find a number of strong dipole-allowed 2p-3d cross sections with peak values near threshold in excess of 10 ' cm', 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-hke 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 N, =3X 1(P cm ' and T, =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. %e 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 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.

We present a unitary equivalent spin-boson Hamiltonian in which terms can be identified which contribute to the Bloch-Siegert shift, and to the level splittings at the anticrossings associated with the Bloch-Siegert resonances. First-order degenerate perturbation theory is used to develop approximate results in the case of moderate coupling for the level splitting.

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