Strong monopole electron-collisional excitation in highly stripped ions

Abstract

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 \ensuremath{\Omega}(Z)=${\ensuremath{\xi}}_{\mathrm{nl}}$(Z)${\mathrm{\ensuremath{\eta}}}_{\mathrm{nl}}$(Z)${\mathrm{N}}_{\mathrm{nl}}$${\mathrm{I}}_{\mathrm{H}/\mathrm{\ensuremath{\Delta}}\mathrm{E}(\mathrm{Z})\phantom{\rule{0ex}{0ex}}}$, where ${\ensuremath{\xi}}_{\mathrm{nl}}$(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 ${\ensuremath{\eta}}_{\mathrm{nl}}$(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 A\r{}. The extension of the monopole excitation scheme to Nd-like ions appears very attractive for longer wavelengths.