Isotope Shift in the Arc Spectrum of Nickel

The isotope shift in 31 spectral lines in the nickel arc spectrum has been determined by the use of a Fabry-Perot interferometer. The normal mass shifts were calculated ({≈}+0.025 cm^-1 between Ni⁵⁸ and Ni⁶⁴) and subtracted from the observed isotope shifts. The differences were attributed to the specific mass and field effects. The relative shifts of levels of four configurations were deduced from the observed line shifts, these being the "complex" configurations 3d⁸4s² and 3d⁸4s4p and the two-electron configurations 3d⁹4s and 3d⁹4p. It was shown that the shifts due to the specific mass effect are a significant part of the observed shifts. Perturbations due to interconfiguration interactions were postulated to explain some of the observed shifts. The isotope shift to be expected between Ni⁵⁸ and Ni⁶⁴ on the basis of field effect calculations is about -0.02 cm^-1 for a single $4s$ electron, while the shifts observed are as large as +0.190 cm^-1. A large fraction of this shift must therefore be attributed to the specific mass effect. By noting the deviations of the relative shifts between adjacent pairs of even isotopes from those predicted by mass effect theory, it was possible to deduce the relative field effect. The relative level shift resulting from the field effect is nearly the same for the adjacent isotope pairs 60-62 and 62-64 while the relative level shift for the isotope pair 58-60 is approximately 0.004 cm^-1 larger than that for the other adjacent isotope pairs. The arrangement of neutrons in the outermost nuclear shells is believed to account for this difference. Within the experimental error the level shift of the Ni⁶¹ relative to the neighboring even isotopes is such that there is no odd-even staggering of the levels.