Relativistic effects in Ni II and the search for variation of the fine-structure constant

Theories unifying gravity and other interactions suggest the possibility of spatial and temporal variation of physical “constants” in the Universe. Detection of high redshift absorption systems intersecting the sight lines toward distant quasars provides a powerful tool for measuring these variations. In the present paper we demonstrate that high sensitivity to variation of the fine-structure constant $\alpha$ can be obtained by comparing cosmic and laboratory spectra of the Ni II ion. Relativistic effects in Ni II reveal many interesting features. The Ni II spectrum exhibits avoided level crossings under variation of $\alpha$ and the intervals between the levels have strong nonlinear dependencies on relativistic corrections. The values of the transition frequency shifts, due to the change of $\alpha ,$ vary significantly from state to state including changes of sign. This enhances the sensitivity to the variation of $\alpha$ and reduces possible systematic errors. Calculations of the $\alpha$ dependence of the nickel ion spectral lines that are detectable in quasar absorption spectra have been performed using a relativistic configuration interaction method.