Measurements of line overlap for resonant photopumping of 4→4 transitions in highly charged nickel-like ions

The wavelength of the 6f7/2{→}3d5/2d transition in several nickel-like ions has been measured on the Livermore electron-beam ion trap with a precision as high as 27 ppm in order to assess their utility in a resonant photopumping scheme to achieve x-ray lasing below 80 {}. An interpolation of the results is presented, which provides an estimate of the transition energies for ions between Dy (Z=66) and U (Z=92). The closest line coincidence was found for the 3d-6f transition in nickel-like Hf, which differs by 1.2 m{} from the wavelength of the Ly-α₂ transition in hydrogenic C1, and for the 3d-6f transition in T1, which differs by 0.8 m{} from Ly-α₁ in hydrogenic Ca.