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 towards distant quasars provide 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 crossing phenomenon 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 change of the sign. This enhances the sensitivity to the variation of alpha and reduces possible systematic errors. The calculations of alpha-dependence of the nickel ion spectral lines that are detectable in quasar absorption spectra have been performed using a relativistic configuration interaction method.