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We study theoretically the electric dipole transitions of even Ni isotopes atlow energies, using the self-consistent quasi-particle random-phaseapproximation (RPA) with the D1S Gogny interaction and a continuum-RPA modelwith the SLy4 Skyrme force. We analyze isoscalar states, isovector states, andthe dipole polarizability. We define a reference value for the polarizability,to remove a trivial dependence on the mass number. We compare our results withdata and other calculations, with a focus on collective states, shell effects,and threshold transitions.  Our results support the presence of a strong isoscalar transition, withlittle or moderate E1 strength, as a universal feature of ordinary nuclei. Inmoderately neutron-rich Ni isotopes, namely 68Ni and neighboring isotopes, thistransition is found bimodal due to couplings with surface neutrons. An adequatetreatment of the continuum states appears essential for describingsuprathreshold E1 strength, especially beyond 68Ni. Very exotic isotopes (N>50)are found highly polarizable, with practically all their E1 strength in thecontinuum. The dipole polarizability and the neutron skin thickness areinfluenced by shell structure in different ways, so they can appearanticorrelated. A comparison with existing results for lighter (Ca) and heavier(Sn) nuclei suggests that the so-called pygmy dipole strength is influencedstrongly by shell effects and that, partly for that reason, its isospinstructure depends on the mass region.

Isoscalar and neutron modes in the E1 spectra of Ni isotopes and the relevance of shell effects and the continuum