Strong monopole electron-collisional excitation in highly stripped ions

The collision strengths for electric monopole collisional excitation of the outermost subshell of highly stripped closed-shell ions are examined and found to scale nearly hydrogenically. The collision strength near threshold for the principal monopole excitation process (in the distorted-wave approximation) is written as Ω(Z)=${\xi }_{\mathrm{nl}}$(Z)${\mathrm{\eta }}_{\mathrm{nl}}$(Z)${\mathrm{N}}_{\mathrm{nl}}$ ${\mathrm{I}}_{\mathrm{H}/\mathrm{\Delta }\mathrm{E}\left(\mathrm{Z}\right)}$, where ${\xi }_{\mathrm{nl}}$(Z) is near 0.3, 0.5, and 0.6 for the He-like (1s–2s), Ne-like (2p–3p), and Ni-like (3d–4d) sequences, respectively, and where ${\eta }_{\mathrm{nl}}$(Z) accounts for mixing effects and is near unity at low Z. The excitation process in nickel-like ions is more effective per electron than for neonlike ions and is favorable to the development of x-ray lasers below 44 Å. The extension of the monopole excitation scheme to Nd-like ions appears very attractive for longer wavelengths.