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Level crossings in the ground state of ions occur when the nuclear charge Zand ion charge Z_ion are varied along an isoelectronic sequence until the twooutermost shells are nearly degenerate. We examine all available levelcrossings in the periodic table for both near neutral ions and highly chargedions (HCIs). Normal E1 transitions in HCIs are in X-ray range, however levelcrossings allow for optical electromagnetic transitions that could form thereference transition for high accuracy atomic clocks. Optical E1 (due toconfiguration mixing), M1 and E2 transitions are available in HCIs near levelcrossings. We present scaling laws for energies and amplitudes that allow us tomake simple estimates of systematic effects of relevance to atomic clocks. HCIclocks could have some advantages over existing optical clocks because certainsystematic effects are reduced, for example they can have much smaller thermalshifts. Other effects such as fine-structure and hyperfine splitting are muchlarger in HCIs, which can allow for richer spectra. HCIs are excellentcandidates for probing variations in the fine-structure constant, alpha, inatomic systems as there are transitions with the highest sensitivity toalpha-variation.

Highly charged ions with E1, M1, and E2 transitions within laser range