Electron collisional excitation in F-like selenium

Abstract

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.

Electron Impact Excitation Cross Sections in F-Like Selenium

“…The oscillator strengths for transitions 2-64 and 2-94 in Tables III-V of Ref. [5] are not in accordance with those of Tables X-XII in that reference. In the present Table III, those oscillator strengths which are smaller than 10 Ϫ4 are set equal to zero.…”

“…Some minor errors in Table I of Ref. [5] have been corrected. In Table II we compare the values of the resonance transition energies obtained by different ways.…”

“…The largest difference is about 0.85 eV, corresponding to the resonance transition to level number 68. The entries labeled "Hagel" show the corresponding relativistic values of Hagelstein [5]. These values were obtained using a mean configuration in determining the central potential used in calculating the radial wave functions.…”

“…In the configuration so characterized, half an electron is excited. The original reason for using a mean configuration with a fictitious occupation number [5,7] is to make all bound and free orbitals orthogonal. From tests mentioned in Ref.…”

“…One perplexing consequence of the observation of Co-like analog lasers is the absence of F-like lasers in Ne-like x-ray laser plasmas. The absence of any measurable gain on F-like lines is currently a mystery [5,6]. Electron impact excitation cross sections for F-like selenium ions have been calculated by Hagelstein [5] using the relativistic distortedwave Born (RDWB) method without exchange potentials, and by Sampson et al [7] using the RDWB method with Dirac-Fock-Slater (DFS) potentials, which are energy independent in the evaluation of continuum orbitals.…”