Wavelengths of the Ni-like 4d 1S0-4p 1P1 x-ray laser line

We measure the wavelengths of the Ni-like 3d(9) 4d S-1(0)-3d(9) 4p P-1(1) x-ray laser line in several low-Z Ni-like ions ranging from Y (Z=39) to Cd (Z=48). These wavelengths are compared with optimized level calculations using a multiconfiguration Dirac-Fock code. With the help of these results, we identify this line to very high accuracy in nonlasing plasmas from As (Z=33) to Mo (Z=42). Accurate values of these wavelengths are essential for performing plasma imaging and interferometry experiments with multilayer optics that use the x-ray laser to backlight other plasmas.-These results also provide important atomic data that are currently missing about the energy of the 4d S-1(0) level in the Ni I sequence. [S1050-2947(98)50809-8].