Electron impact excitation of Helium-like ions up to n=4 levels including radiation damping

Abstract

Helium-like ions provide the most important X-ray spectral diagnostics in high temperature fusion and astrophysical plasmas. We previously presented computed collision strengths for O~VII including relativistic fine structure, levels up to the $n=4$ complex and radiation damping of autoionizing resonances. We have extended this work to other He-like ions (N, Ne, Mg, Al, Si, S, Ca). The calculations are carried out using the Breit-Pauli R-matrix (BPRM) method with a 31-level eigenfunction expansion. Collision strengths for the principal lines important in X-ray plasma diagnostics, w, x, y and z, corresponding to the 4 transitions to the ground level 1s^2(^1S_0) <- 1s2p(^1P^o_1), 1s2p(^3P^o_2), 1s2p(^3P^o_1), 1s2s(^3S_1), are explicitly shown. We find the effect of radiation damping to be significant for the forbidden transitions in heavier He-like ions, which should affect the diagnostic line ratios. We extrapolated the collision strengths to their values at infinite energy using the Burgess-Tully extrapolation technique. This is required to calculate the Maxwellian average collision strengths at high temperature. We show that the coupling between dipole allowed and inter-combination transitions affects increasingly the effective collision strengths for the n ^1S_0 - n' ^3P_1 transition as the charge of the ion increases. This clearly affects the treatment of the extrapolation toward the infinite energy point of the collision strength. This work is carried out as part of the Iron Project-RmaX Network.

arXiv comment13 pages, 6 figures, submitted to J.Phys.B

References in corpus (5)

• X-ray photoionized plasma diagnostics with Helium-like ions. Application to Warm Absorber-Emitter in Active Galactic Nuclei
• Line ratios for Helium-like ions: Applications to collision-dominated plasmas
• Multipole (E1, M1, E2, M2) transition wavelengths and rates between states with n<= 6 in heliumlike carbon, nitrogen, oxygen, neon, silicon, and argon
• Self-Consistent R-matrix Approach To Photoionization And Unified Electron-Ion Recombination
• Electron impact excitation of helium-like oxygen up to n = 4 levels including radiation damping

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