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) = ξ/sub n//sub l/(Z)eta/sub nl/(Z)N/sub nl/I/sub H//ΔE(Z), where ξ/sub n//sub l/(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/sub n//sub l/(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. The extension of the monopole excitation scheme to Nd-like ions appears very attractive for longer wavelengths